Combustion exhaust gas treatment system

ABSTRACT

Five combustion exhaust gas treatment systems capable of removing dust and selenium (Se) in combustion exhaust gas and making harmless are proposed: (1) combustion exhaust gas is cooled to 350° C. or less, dust is separated, Se is transformed into insoluble compound, and Se is separated; (2) combustion exhaust gas is cooled to 350° C. or less, dust is separated, Se elution preventive agent is added, and dust is formed into scale; (3) dust in combustion exhaust gas is collected by dust collector, dust is formed into slurry by making use of part of circulation liquid in desulfurization apparatus, and tetravalent Se in dust slurry is transformed into insoluble compound, which is separated into solid and liquid; (4) dust is separated from combustion exhaust gas by dust collector, and it is heated to gasify Se, and is led into desulfurization apparatus, etc., and formed into slurry, and Se is made into insoluble compound; and (5) a treating agent for making tetravalent Se insoluble is added to part of circulation liquid in desulfurization apparatus, and it is sprayed into combustion exhaust gas to make Se insoluble.

SPECIFICATION

This is a divisional of prior U.S. patent application Ser. No.09/059,675 filed Apr. 13, 1998 now abandoned, which is a divisional ofprior U.S. patent application Ser. No. 08/592,116 filed Jan. 26, 1996now U.S. Pat. No. 6,113,863.

FIELD OF THE INVENTION AND RELATED ARTS

The present invention relates to a combustion exhaust gas treatmentsystem, more particularly to a combustion exhaust gas treatment systemcapable of easily removing dust and selenium (Se) in combustion exhaustgas containing dust and Se components, and making harmless.

Hitherto, as the combustion exhaust gas treatment system installed in athermal power plant or the like, a combustion exhaust gas treatmentsystem comprising a dust collector (usually an electrostaticprecipitator) for removing fly ash and other dust from the combustionexhaust gas, and a combustion exhaust gas desulfurization apparatus forabsorbing sulfurous acid in the combustion exhaust gas is generallyemployed.

Moreover, conventionally, as the combustion exhaust gas treatment systeminstalled in a thermal power plant or the like, a combustion exhaust gastreatment system comprising a dry dust collector (usually anelectrostatic precipitator) for removing fly ash and other dust from thecombustion exhaust gas, and a wet combustion exhaust gas desulfurizationapparatus for absorbing sulfurous gas in the combustion exhaust gas bycontacting with an absorbent slurry (for example, slurry containingcalcium compound) in the absorption column, and separating andrecovering gypsum as byproduct from the slurry in the absorption columnis generally employed.

Recently, handling of harmful impurities contained in the combustionexhaust gas aside from sulfur oxides is posing a problem. In particular,in the combustion exhaust gas treatment system for coal fired boiler,the harmfulness of selenium (Se) contained at a maximum level of about10 mg/kg in coal is a problem lately, and its harmless treatment isdemanded.

Meanwhile, Se exists as tetravalent Se (main form: selenious acid SeO₃²⁻) which is easy to treat by making insoluble by treating agent, andhexavalent Se (main form: selenic acid SeO₄ ²⁻) which is hard to treatby making insoluble, and in particular the hexavalent Se is high insolubility (solubility at 20° C. is 95%) and is easy to elute. Besides,this Se has a toxicity similar to that of arsenic compound, and disastercases and emission regulations are known overseas, and it is newly addedto the list of regulated items also in Japan, and is controlled by theenvironmental standard (0.01 mg/liter), discharge standard (0.1mg/liter), and elution standard in landfill (0.3 mg/liter).

FIG. 24 shows an example of prior art of combustion exhaust gastreatment system of this type (an example of combustion exhaust gastreatment system for coal fired boiler). In FIG. 24 and FIG. 25, thecombustion exhaust gas A emitted from a coal fired boiler 1 is sent intoa denitration apparatus 2 installed downstream of the boiler 1 to be ridof nitrogen oxides (NOx), and passes through an air heater 3 and a heatrecovery unit 4 of gas-gas heater (GGH), and is introduced into anelectrostatic precipitator (EP) 5, in which fly ash and dust areremoved. In succession, the combustion exhaust gas is guided into a wetcombustion exhaust gas desulfurization apparatus 7 by a fan 6, andsulfurous gas is removed in this desulfurization apparatus 7, and afterpassing through a reheater 8 of the gas-gas heater (GGH), it is led intoa stack 10 by a fan 9, and is released into the atmosphere through thestack 10 (FIG. 25).

On the other hand, fly ash and dust removed in the electrostaticprecipitator 5 are discharged from plural hoppers 5 a (dust recoveryunits) formed in the electrostatic precipitator 5, and are conveyed andcollected in batch by a conveyor 11. Thus collected dust B is eitherrecycled as cement material or the like, or discarded in an ash disposalyard (FIG. 24).

Herein, the desulfurization apparatus 7 comprises an absorption column,for example, in which combustion exhaust gas is introduced, and bycontact of combustion exhaust gas with absorbent slurry (usually slurrycontaining calcium compound) in this absorption column, the sulfurousacid in the combustion exhaust gas is absorbed in wet process, andusually from the slurry in the absorption column, gypsum is separatedand collected as byproduct.

Incidentally, the heat recovery unit 4 of the gas-gas heater (GGH) maybe also disposed immediately before the desulfurization apparatus 7 asshown in FIG. 26.

In these combustion exhaust gas treatment systems, most of Se in coal(Se in combustion exhaust gas) is condensed at the downstream side ofthe air heater 3 (that is, the position before introduction into theelectrostatic precipitator 5), and is removed by the electrostaticprecipitator 5 in a state being contained in the dust in combustionexhaust gas, and is directly mixed in the refuse in the ash disposalyard or in the cement material. To render Se harmless by conforming tothe elution standard, it requires a complicated and costlyaftertreatment of, for example, diluting the ash removed by theelectrostatic precipitator 5 in a huge volume of water.

OBJECTS AND SUMMARY OF THE INVENTIONS

In the light of the prior arts, it is an object of the invention topresent a combustion exhaust gas treatment system capable of easilyremoving and making harmless the Se components contained in combustionexhaust gas.

To achieve the object, five inventions are proposed.

A. First Invention:

(1) A combustion exhaust gas treatment system for treating combustionexhaust gas containing dust and Se components, comprising means forcooling combustion exhaust gas to 350° C. or less, dust collecting meansfor separating dust in the combustion exhaust gas, and Se treating meansfor transforming the existent form of Se in the dust into an insolublecompound by adding water and treating agent to the dust separated by thedust collecting means.

(2) A combustion exhaust gas treatment system for treating combustionexhaust gas containing dust and Se components, comprising means forcooling combustion exhaust gas to 350° C. or less, dust collecting meansfor separating dust in the combustion exhaust gas, Se treating means fortransforming the existent form of Se in the dust into an insolublecompound by adding water and treating agent to the dust separated by thedust collecting means, and solid-liquid separating means for separatingthe slurry containing insoluble Se compound discharged from the Setreating means into solid and liquid.

Preferred embodiments of the first invention includes the followingcombustion exhaust gas treatment systems (3) to (6).

(3) A combustion exhaust gas treatment system of (1) or (2), wherein thedust collecting means is a dust collecting apparatus constituted byforming a plurality of recovery units for separating and collecting dustfrom the inlet side to the outlet side of the combustion exhaust gas,separating and collecting the dust collected from the recovery unit atthe inlet side of the combustion exhaust gas and the dust collected atthe outlet side separately, and introducing only the dust collected atthe outlet side into the Se treating means.

(4) A combustion exhaust gas treatment system of (1) or (2), furthercomprising sorting means for sorting the dust separated by the dustcollecting means into large particle size group and small particle sizegroup, wherein only the small particle size dust sorted by the sortingmeans is introduced into the Se treating means to make Se insoluble.

(5) A combustion exhaust gas treatment system of (3) or (4), furthercomprising means for mixing the dust making Se insoluble by the Setreating means, and the remaining dust not making Se insoluble, so thatthe moisture content may be 20% or less.

(6) A combustion exhaust gas treatment system of (5), further comprisingmeans for forming the dust mixed by the mixing means into scale.

In the combustion exhaust gas treatment system of the first invention,the method of making Se insoluble may be realized in various forms, andrepresentative forms include a method of forming slurry by adding waterto the dust containing Se components separated and recovered by the dustcollecting means, adding treating agent to the slurry to make Seinsoluble, and separating into solid and liquid, and a method ofspraying a solution of treating agent to the dust to immerse uniformly,and making Se insoluble.

In the combustion exhaust gas treatment system of the first invention,most of the Se in the combustion exhaust gas is condensed as beingcooled by the cooling means, and is removed by the dust collecting meansin a state being contained in the dust. In consequence, the treatingagent is added by the Se treating means to the dust separated by thedust collecting means, and the existent form of the Se in dust istransformed into an insoluble compound.

Accordingly, if the dust is discarded same as in the prior art, the Seelution standard is satisfied, and the Se is made harmless easilywithout requiring complicated aftertreatment.

By making Se insoluble only in the dust separated and collected from thespecific recovery unit at the outlet side of the combustion exhaust gasin the dust collecting means, the required amount of treating agent andcapacity of Se treating means can be reduced, and the Se is madeharmless easily and at low cost.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash) separated and collected from the specific recovery unit at theoutlet side, and the Se is made harmless on the whole only by applyinginsoluble treatment on the dust of smaller particle size.

Moreover, by the same reason, by making Se insoluble only in the dust ofsmaller particle size sorted by the sorting means, the required amountof treating agent and capacity of Se treating means can be reduced, andthe Se is made harmless more easily and economically.

Incidentally, in the case of the apparatus for making Se insoluble onlyin part of the dust, by further comprising the mixing means, the dustmaking Se insoluble, and the remaining dust without Se insolubletreatment are mixed so that the moisture content may be 20% or less, andtherefore the moisture content in the dust can be easily lowered andhandling is made easier in the disposal process of dust, withoutinstalling any huge equipment such as solid-liquid separator thatrequires wastewater (filtrate) treatment.

Moreover, by further comprising means for making the dust mixed by themixing means into scale form, handling in dust disposal process is mucheasier.

The combustion exhaust gas treatment system of the first invention isintended to separate Se components contained in the combustion exhaustgas from the combustion exhaust gas together with dust, and makingharmless by further making it insoluble. The dust being rid of dustincluding Se components is further led into the wet combustion exhaustgas desulfurization apparatus, and SO₂ is removed, and is madecompletely harmless by proper treatment by ordinary method, and isreleased into the atmosphere.

B. Second Invention:

According to the system by the first invention, although the Se in theflue can be easily made harmless, but in order to form slurry by addingwater to the separated and collected dust, and to make Se insoluble byadding and mixing insoluble treating agent to separated into solid andliquid, it requires wastewater treatment apparatus, solid-liquidseparating apparatus, and other devices.

A second invention presents a combustion exhaust gas treatment systemcapable of easily making harmless the Se contained in the combustionexhaust gas.

(1) A combustion exhaust gas treatment system for removing harmfulmatter in combustion exhaust gas, comprising means for coolingcombustion exhaust gas to 350° C. or less, dust collecting means forseparating dust in the combustion exhaust gas, and mixing means foradding and mixing Se elution preventive agent and humidifying liquid orsolution of Se elution preventive agent to the dust separated by thedust collecting means.

(2) A combustion exhaust gas treatment system for removing harmfulmatter in combustion exhaust gas, comprising means for coolingcombustion exhaust gas to 350° C. or less, dust collecting means forseparating dust in the combustion exhaust gas, mixing means for addingand mixing Se elution preventive agent and humidifying liquid orsolution of Se elution preventive agent to the dust separated by thedust collecting means, and means for forming the dust mixed with the Seelution preventive agent and humidifying agent or solution of Se elutionpreventive agent by the mixing means into scale form.

Preferred embodiments of the second invention involve the combustionexhaust gas treatment systems (3) and (4).

(3) A combustion exhaust gas treatment system of (1) or (2), wherein thedust collecting means is a dust collecting apparatus constituted byforming a plurality of recovery units for separating and collecting dustfrom the inlet side to the outlet side of the combustion exhaust gas,separating and collecting the dust collected from the recovery unit atthe inlet side of the combustion exhaust gas and the dust collected atthe outlet side separately, and introducing only the dust collected atthe outlet side into the mixing means.

(4) A combustion exhaust gas treatment system of (1) or (2), furthercomprising sorting means for sorting the dust separated by the dustcollecting means into large particle size group and small particle sizegroup, wherein only the small particle size dust sorted by the sortingmeans is introduced into the mixing means.

In the combustion exhaust gas treatment system of the first invention,most of the Se in the combustion exhaust gas is condensed as beingcooled by the cooling means, and is removed by the dust collecting meansin a state being contained in the dust. In consequence, the Se elutionpreventive agent and humidifying liquid or solution of Se elutionpreventive agent are added by the mixing means to the dust separated bythe dust collecting means, and the existent form of the Se in dust istransformed into an insoluble compound. Accordingly, if the dust isdiscarded same as in the prior art, the Se elution standard issatisfied, and the Se is made harmless easily without requiringcomplicated aftertreatment. Still more, because of the constitution ofadding humidifying liquid, Se elution preventive agent or solution of Seelution preventive agent, and mixing the dust and Se elution preventiveagent, or further forming the mixture into scale, as compared with theconstitution of forming the dust into slurry and mixing Se insolubletreating agent and the separating into solid and liquid and discarding,it does not require large equipment or apparatus for wastewater(filtrate) treating facility or solid-liquid separator, and handling indisposal of dust is much easier.

By making insoluble only in the dust separated and collected from thespecific recovery unit at the outlet side of the combustion exhaust gasin the dust collecting means by introducing into the mixing means, therequired amount of Se elution preventive agent and capacity of mixingmeans and scale forming means can be reduced, and the Se is madeharmless easily and at low cost.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash) separated and collected from the specific recovery unit at theoutlet side, and the Se is made harmless on the whole only by applyinginsoluble treatment on the dust of smaller particle size.

Moreover, by the same reason, by making Se insoluble only in the dust ofsmaller particle size sorted by the sorting means, the required amountof treating agent and capacity of Se treating means can be reduced, andthe Se is made harmless more easily and economically.

C. Third Invention:

(1) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dry dust collector for removing dust in the combustionexhaust gas, a desulfurization apparatus having a desulfurization columnfor circulating an absorbent slurry for absorbing and removing sulfurousacid, repulping means for dissolving the dust removed by the dry dustcollector in water to form into slurry, treating agent feeding means forfeeding a treating agent for making tetravalent Se insoluble into thedust slurry obtained in the repulping means, means for separating thedust slurry containing Se made insoluble into solid and liquid, and apiping system for introducing the separation liquid discharged from theseparating means into the absorbent slurry.

(2) A combustion exhaust gas treatment system of (1), wherein part ofthe circulating liquid forming the slurry of the desulfurization systemis introduced into the repulping means, and is used as the solvent inthe repulping means.

(3) A combustion exhaust gas treatment system of (1) or (2), furthercomprising means for feeding filter additive to the dust slurry.

(4) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dry dust collector for removing dust in the combustionexhaust gas, a desulfurization apparatus having a desulfurization columnfor circulating an absorbent slurry for absorbing and removing sulfurousacid, means for introducing the dust removed by the dry dust collectorinto the absorbent slurry, and treating agent feeding means for feedinga treating agent for making tetravalent Se insoluble into the absorbentslurry.

(5) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a desulfurization apparatus having a desulfurization columnfor circulating an absorbent slurry for absorbing and removing sulfurousacid in the combustion exhaust gas, and treating agent feeding means forfeeding a treating agent for making tetravalent Se insoluble into theabsorbent slurry, wherein the combustion exhaust gas is introduceddirectly into the desulfurization column.

(6) A combustion exhaust gas treatment system of any one of (1) to (5),further comprising oxidation-reduction reaction control means forcontrolling the oxidation-reduction reaction in the desulfurizationapparatus, so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be reduced by the sulfurous acid in theslurry to be tetravalent.

(7) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dry dust collector for removing dust in the combustionexhaust gas, a desulfurization apparatus having a cooling and dustcollecting column disposed upstream of an absorption column, andpossessing a desulfurization column for circulating an absorbent slurryfor absorbing and removing sulfurous acid, means for feeding the dustremoved by the dry dust collector into the circulating slurry in thecooling and dust collecting column, and means for feeding a treatingagent for making tetravalent Se insoluble into the circulating slurry inthe cooling and dust collecting column.

(8) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a desulfurization apparatus having a cooling and dustcollecting column disposed upstream of an absorption column, andpossessing a desulfurization column for circulating an absorbent slurryfor absorbing and removing sulfurous acid, and means for feeding atreating agent for making tetravalent Se insoluble into the circulatingslurry in the cooling and dust collecting column, wherein the combustionexhaust gas is introduced directly into the cooling and dust collectingcolumn.

(9) A combustion exhaust gas treatment system of any one of (1) to (8),further comprising a wastewater treatment apparatus for treatingwastewater from the desulfurization apparatus, and treating agentfeeding means for feeding a treating agent for making tetravalent Seinsoluble to the impurity slurry separated by this wastewater treatmentapparatus.

Any combustion exhaust gas treatment system of the second inventioncomprises means for feeding a treating agent for making insoluble thetetravalent Se dissolved in the slurry. The treating agent for makingtetravalent Se insoluble includes, for example, FeCl₃, Fe₂( SO₄)₃,chelating agent (e.g. Epolus MX-7 of Miyoshi Resin), and high molecularheavy metal capturing agent (e.g. Epofloc L-1 of Miyoshi Resin). Thereaction of these treating agent for making insoluble tetravalent Se isdescribed later.

In the combustion exhaust gas treatment system of (1), most of Se in thecombustion exhaust gas is removed by the dry dust collector in a statebeing included in the dust, and the dust contacts with water or liquid(solvent) in the repulping means, and is dissolved into the liquid inthe process of forming into slurry. As the slurry forming liquid(solvent), aside from the water supplied from outside the system, thetreated water or slurry coming from each process in the system may beused. Of the Se components dissolved in the dust slurry, at leasttetravalent Se is made insoluble by treating agent, and is discharged tothe solid phase side by the separating means (into the dust cake). Onthe other hand, the filtrate separated by the separating means isintroduced into the slurry in the desulfurization apparatus.Accordingly, hexavalent Se is also included in the combustion exhaustgas, and if it is not made insoluble by the treating agent and isdissolved in the filtrate, most of the hexavalent Se is introduced intothe slurry in the desulfurization apparatus, and reacts with thesulfurous acid absorbed from the combustion exhaust gas into the slurry,and is reduced to return to tetravalent Se.

Therefore, if hexavalent Se is contained in the combustion exhaust gas,in the stationary state, Se components dissolved in the slurry in thedesulfurization apparatus or the circulation liquid composing thisslurry are mainly tetravalent Se components, and in the wastewatertreatment apparatus for treating discharge of circulating liquid of thedesulfurization apparatus, only by making this tetravalent Se insoluble,the Se elution standard may be easily satisfied, and the desulfurizationapparatus can be used as reduction reaction apparatus for hexavalent Se,so that the constitution of the entire system may be simplified.

In the combustion exhaust gas treatment system of (2), since part of thecirculating liquid for composing the slurry of the desulfurizationapparatus in the combustion exhaust gas treatment system of (1) is usedas solvent in the repulsing means, as compared with the constitution offeeding water separately, the quantity (circulation) and consumption ofwater can be reduced.

In the combustion exhaust gas treatment system of

(3), since filter additive is charged into the mixing means orseparating means, the dehydrating performance in the separating means isenhanced, and a solid matter (dust cake) low in moisture content andeasy to handle is obtained. As the filter additive, gypsum used in thedesulfurization process or the like may be used.

In the combustion exhaust gas treatment system of (4), most of Se incombustion exhaust gas is removed by the dry dust collector in a statebeing contained in the dust, and is directly led into the slurry in thedesulfurization apparatus, repulped in the desulfurization apparatus,and mixed with a treating agent for making insoluble. Accordingly, atleast tetravalent Se components of the Se components contained in thecombustion exhaust gas are mostly made insoluble directly by thetreating agent in the desulfurization apparatus, and mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus and discharged, and the remaining Secomponents are also easily made insoluble and solidified by theinsoluble treating agent in the wastewater treatment apparatus fortreating discharge of circulating liquid in the desulfurizationapparatus. Besides, if hexavalent Se components are contained in thecombustion exhaust gas, most of hexavalent Se reacts with the sulfurousacid absorbed from the combustion exhaust gas in the slurry in thedesulfurization apparatus to be reduced to change to tetravalent Se,which is also made insoluble by the treating agent in thedesulfurization apparatus and is mixed into the solid matter (gypsum,etc.) separated and formed from the slurry in the desulfurizationapparatus and discharged, or easily made insoluble and solidified in thewastewater treatment apparatus.

Therefore, in this treatment system, too, the Se elution standard can beeasily satisfied, and moreover since the desulfurization apparatusfunctions both as reducing reaction apparatus of hexavalent Se and asrepulping means of dust, the constitution of the entire system may befurther simplified.

In the combustion exhaust gas treatment system of (5), most of Se incombustion exhaust gas is directly introduced into the desulfurizationapparatus together with the combustion exhaust gas in a state beingcontained in dust, and is mixed into the absorbent slurry in, forexample, the absorption column of the desulfurization apparatus, and isrepulped and mixed with the treating agent. Accordingly, at leasttetravalent Se components of the Se components contained in thecombustion exhaust gas are mostly made insoluble directly by thetreating agent in the desulfurization apparatus, and mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus and discharged, or easily made insoluble andsolidified by adding an insoluble treating agent in the wastewatertreatment apparatus for treating discharge of circulating liquid in thedesulfurization apparatus. Besides, if hexavalent Se components arecontained in the combustion exhaust gas, most of hexavalent Se reactswith the sulfurous acid absorbed from the combustion exhaust gas in theslurry in the desulfurization apparatus to be reduced to change totetravalent Se, which is also made insoluble by the treating agent inthe desulfurization apparatus and is mixed into the solid matter(gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus and discharged, or easily made insoluble andsolidified in the wastewater treatment apparatus.

Therefore, in this treatment system, too, the Se elution standard can beeasily satisfied, and moreover since the desulfurization apparatusfunctions as the dust collector, reducing reaction apparatus ofhexavalent Se, and repulping means of dust, the constitution of theentire system may be further simplified, as compared with theconstitution for installing the dust collector, repulping means andothers separately.

In the combustion exhaust gas treatment system of (6), theoxidation-reduction reaction control means controls theoxidation-reduction reaction in the desulfurization apparatus so thatthe hexavalent Se mixing in the slurry in the desulfurization apparatusmay be reduced almost completely by sulfurous acid in the slurry to betetravalent. Accordingly, if hexavalent SE is contained in thecombustion exhaust gas, this hexavalent Se can be almost completelychanged into tetravalent form in the desulfurization apparatus, so thatthe Se in the combustion exhaust gas may be made insoluble more easilyand completely.

In the combustion exhaust gas treatment system of (7), most of Se incombustion exhaust gas is removed by the dry dust collector in a statebeing contained in the dust, and is directly introduced into the liquidin the cooling and dust collecting column of the desulfurizationapparatus, and is repulped in the cooling and dust collecting column. Inthe dust slurry formed by feeding the dust containing Se into the liquidin the cooling and dust collecting column and repulsing, a treatingagent for making tetravalent Se insoluble is mixed. Accordingly, atleast tetravalent Se components of Se components contained in thecombustion exhaust gas are directly made insoluble by the treating agentin the desulfurization apparatus, and discharged into the solid-phaseside by separating means (into the dust cake), or are easily madeinsoluble by addition of insoluble treating agent in the subsequenttreatment of the separated water, so as to be discharged. Ig hexavalentSe components are contained in the Se components in the combustionexhaust gas, most of hexavalent Se components react with sulfurous acidabsorbed from the combustion exhaust gas into the liquid in the coolingand dust collecting column to changed to reduced tetravalent Secomponents, which are also made insoluble by the treating agent, anddischarged into the solid-phase side by separating means (into the dustcake), so as to be treated to be harmless.

Therefore, in this treatment system, too, the Se elution standard may beeasily satisfied. Moreover, since the desulfurization apparatusfunctions also as reducing reaction apparatus of hexavalent Se or asrepulping means of dust, the constitution of the entire system may befurther simplified. In this treatment system, since dust is not mixedinto the slurry in the absorption column of the desulfurizationapparatus, the performance of the desulfurization apparatus such asdesulfurization rate may be maintained high.

In the combustion exhaust gas treatment system of (8), most of Se incombustion exhaust gas is directly fed in the cooling and dustcollecting column of the desulfurization apparatus together with thecombustion exhaust gas in a state being contained in the dusts in thecombustion exhaust gas, and is repulped in this cooling and dustcollecting column. In the dust slurry repulped as the dust containing Seis fed into the liquid in the cooling and dust collecting column, atreating agent for making tetravalent Se insoluble is mixed.Accordingly, at least tetravalent Se components of Se componentscontained in the combustion exhaust gas are directly made insoluble bythe treating agent in the desulfurization apparatus, and discharged tothe solid-phase side (in the dust cake) by the separating means forseparating the dust slurry into solid and liquid, or is easily madeinsoluble by the addition of insoluble treating agent in the subsequenttreatment of separated water so as to be discarded. If hexavalent Secomponents are contained in the combustion exhaust gas, most of thesehexavalent Se components react with the sulfurous acid absorbed from thecombustion exhaust gas in the liquid in the cooling and dust collectingcolumn to transform to reduced tetravalent Se, which is also madeinsoluble by the treating agent, and is discharged to the solid-phaseside (in the dust cake) by the separating means, and is made harmless.

Therefore, in this treatment system, too, the Se elution standard can beeasily satisfied, and moreover since the desulfurization apparatusfunctions as the dust collector, reducing reaction apparatus ofhexavalent Se, and repulping means of dust, the constitution of theentire system may be further simplified, as compared with theconstitution for installing the dust collector, repulping means andothers separately. In addition, in this treatment system, dust is notmixed into the slurry in the absorption column of the desulfurizationapparatus, and the performance of the desulfurization apparatus such asdesulfurization rate may be maintained high.

In the combustion exhaust gas treatment system of (9), in a prior stageof treatment by solidifying the impurities in the wastewater treatingdevice in the desulfurization apparatus, a treating agent for makingtetravalent Se insoluble is added, and the Se eluting in the wastewatercan be made insoluble to change into disposable form. In the combustionexhaust gas treatment system of (1) to (8), Se in the combustion exhaustgas can be almost completely made insoluble, but this system iseffective in the case where Se is not to be made insoluble, for example,it is necessary to treat Se concentrated in the circulation, beingdissolved in the absorption liquid slurry due to reduction of hexavalentSe, mixing of Se into the gypsum to be collected in the desulfurizationprocess must be avoided. In the apparatus of (4) to (8), depending onthe conditions of treatment, the insoluble treating agent may besupplied only in the wastewater treating apparatus.

D. Fourth Invention:

(1) A combustion exhaust gas treatment system for treating combustionexhaust gas containing dust and Se components, comprising a dustcollector for removing dust from the combustion exhaust gas, and meansfor heating the dust removed by the dust collector to a temperature forgasification of Se in the dust.

(2) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dust collector for removing dust from the combustionexhaust gas, a desulfurization apparatus having an absorption column forcirculating an absorbent slurry for absorbing and removing sulfurousacid, and means for heating the dust removed by the dust collector to atemperature for gasification of Se in the dust, wherein the gasgenerated by heating the dust by the heating means is fed into thedesulfurization apparatus together with the combustion exhaust gas, andSe is dissolved and captured in the slurry in the desulfurizationapparatus, and a treating agent for making the tetravalent Se insolubleis mixed in the treating process of slurry.

(3) A combustion exhaust gas treatment system of (2), further comprisingoxidation-reduction reaction control means for controlling theoxidation-reduction reaction in the desulfurization apparatus, so thatthe hexavalent Se mixed in the slurry in the desulfurization apparatusmay be reduced by sulfurous acid in the slurry to be tetravalent Se.

(4) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dust collector for removing dust from the combustionexhaust gas, a desulfurization apparatus having an absorption column forcirculating an absorbent slurry for absorbing and removing sulfurousacid and a cooling and dust collecting column disposed upstream of theabsorption column, and means for heating the dust removed by the dustcollector to a temperature for gasification of Se in the dust, whereinthe gas generated by heating the dust by the heating means is fed intothe desulfurization apparatus together with the combustion exhaust gas,and Se is dissolved and captured in the circulation liquid in thecooling and dust collecting column, and a treating agent for making thetetravalent Se insoluble is mixed in the treating process of thecirculation liquid.

(5) A combustion exhaust gas treatment system of any one of (1) to (4),wherein the dust collecting means comprises a plurality of recoveryunits for separating and collecting dust, from the inlet side to theoutlet side of the combustion exhaust gas, the dust collected from therecovery unit at the inlet side of the combustion exhaust gas and thedust collected from the outlet side are individually separated andcollected, and only the dust separated and collected from the recoveryunit at the outlet side is fed into the heating means.

(6) A combustion exhaust gas treatment system of any one of (1) to (4),further comprising sorting means for sorting the dust separated by thedust collecting means into large particle size group and small particlesize group, wherein only the small particle size dust sorted by thesorting means is introduced into the heating means.

(7) A combustion exhaust gas treatment system of any one of (1) to (6),wherein the heating means is capable of heating the dust to anytemperature in a range of 100 to 1200° C.

In the combustion exhaust gas treatment system of (1), most of Se incombustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. Accordingly, almost no Se is left over in the dust aftertreatment, and the Se elution standard is satisfied, and hence it can bedirectly recycled as cement material or discarded.

In the combustion exhaust gas treatment system of (2), most of Se incombustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. The gasified Se is fed into the desulfurization apparatustogether with the combustion exhaust gas being rid of dust, and isdissolved and captured in the absorbent slurry. In the treating processof the absorbent slurry, it is mixed with a treating agent for makingtetravalent Se insoluble, and is made insoluble. That is, at leasttetravalent Se is directly made insoluble by the treating agent in thedesulfurization apparatus, and is discharged as being mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus, or is made insoluble by the treating agent inthe wastewater treating apparatus for treating the discharge ofcirculation liquid in the desulfurization apparatus, and is easilysolidified.

Therefore, when the content of hexavalent Se is small in the absorptionliquid in the desulfurization apparatus, only by making tetravalent Seinsoluble, the Se elution standard can be satisfied without releasing Seinto the atmosphere. Moreover, in the constitution where Se is separatedfrom the dust by the heating means and is introduced into thedesulfurization apparatus, without feeding the entire dust into thedesulfurization apparatus, the dust can be recycled easily, and loweringof desulfurization performance in the desulfurization apparatus can beavoided.

In the combustion exhaust gas treatment system of (3), theoxidation-reduction reaction control means controls theoxidation-reduction reaction of the slurry in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost completely reduced by thesulfurous acid in the slurry to become tetravalent. Accordingly, thehexavalent Se can be changed to tetravalent almost completely in thedesulfurization apparatus, and the Se in combustion exhaust gas can beeasily and completely made insoluble.

In the combustion exhaust gas treatment system of (4), most of Se incombustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. The gasified Se is fed into the cooling and dustcollecting column of the desulfurization apparatus together with thecombustion exhaust gas being rid of dust, and is dissolved and capturedin the circulation liquid. In the treating process of the circulationliquid, it is mixed with a treating agent for making tetravalent Seinsoluble, and is made insoluble. That is, at least tetravalent Se isdirectly discharged to the solid-phase side by solid-liquid separatingmeans or the like connected to the cooling and dust collecting column ofthe desulfurization apparatus, or is made insoluble by the treatingagent in the wastewater treating apparatus for treating the discharge ofcirculation liquid in the desulfurization apparatus, and is easilysolidified. Besides, most hexavalent Se reacts with sulfurous acidabsorbed from the combustion exhaust gas in the liquid in the coolingand dust collecting column and is reduced to be tetravalent Se, and ismade insoluble by the treating agent, and is discharged to thesolid-phase side by the separating means so as to be made harmless.

Therefore, in this system, too, the Se elution standard can be satisfiedeasily without releasing Se into the atmosphere, and the cooling anddust collecting column of the desulfurization apparatus functions alsoas hexavalent Se reduction reaction facility, so that the constitutionof the entire system may be simplified. Also in this system, Se or otherdust rarely mixes into the slurry in the absorption column of thedesulfurization apparatus, and the desulfurization performance in thedesulfurization apparatus can be maintained high, and moreover gypsum ofhigh quality can be collected as byproduct.

In the combustion exhaust gas treatment system of (5), only the dustseparated and collected from a specific recovery unit at the outlet sideof the combustion exhaust gas in the dust collecting means is fed intothe heating means, and Se is gasified and removed, and therefore therequired capacity of the heating means may be reduced. Moreover, in thesubsequent desulfurization apparatus and others, the required amount ofthe treating agent for making Se insoluble is also reduced, so that Seis made harmless more easily and inexpensively.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash) separated and collected from the specific recovery unit at theoutlet side, and the Se is made harmless on the whole only by heatingdust of smaller particle size, and applying insoluble treatment ongasified Se.

In the combustion exhaust gas treatment system of (6), Se is gasifiedand separated by feeding only the dust of small particle size sorted bythe sorting means into the heating means, and therefore the requiredcapacity of the heating means may be reduced. Moreover, in thesubsequent desulfurization apparatus and others, the required amount ofthe treating agent for making Se insoluble is also reduced, so that Seis made harmless more easily and inexpensively.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash), and the Se is made harmless on the whole only by heating dust ofsmaller particle size, and applying insoluble treatment on gasified Se.

In the combustion exhaust gas treatment system of (7), the heatingtemperature of dust by the heating means is 100 to 1200° C., andtherefore recondensation of gasified Se into dust is prevented, and Secan be easily removed from dust, and the Se elution standard of dust issatisfied.

E. Fight Invention:

(1) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dust collector for removing dust in the combustion exhaustgas, a desulfurization apparatus having an absorption column forcirculating an absorbent slurry for absorbing and removing sulfurousacid, means for mixing a treating agent for making at least tetravalentSe insoluble, to a circulation liquid composing the absorbent slurryextracted from the desulfurization apparatus, and means for spraying thecirculation liquid mixed with the treating agent by the mixing agent,into a combustion exhaust gas lead-in passage upstream of the dustcollector.

(2) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a desulfurization apparatus having an absorption column forcirculating an absorbent slurry for absorbing and removing sulfurousacid in the combustion exhaust gas, means for mixing a treating agentfor making at least tetravalent Se insoluble, to a circulation liquidcomposing the absorbent slurry extracted from the desulfurizationapparatus, and means for spraying the circulation liquid mixed with thetreating agent by the mixing agent, into a combustion exhaust gaslead-in passage upstream of the desulfurization apparatus, wherein thecombustion exhaust gas is directly fed into the desulfurizationapparatus.

(3) A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a desulfurization apparatus having an absorption column forcirculating an absorbent slurry for absorbing and removing sulfurousacid in the combustion exhaust gas, with a cooling and dust collectingcolumn disposed upstream of the absorption column, means for separatingthe circulation slurry extracted from the cooling and dust collectingcolumn into solid and liquid, means for mixing a treating agent formaking at least tetravalent Se insoluble, to the separated liquiddischarged from the separating means, and means for spraying theseparated liquid mixed with the treating agent by the mixing agent, intoa combustion exhaust gas lead-in passage upstream of the desulfurizationapparatus, wherein the combustion exhaust gas is directly fed into thecooling and dust collecting column of the desulfurization apparatus.

(4) A combustion exhaust gas treatment system of any one of (1) to (3),further comprising oxidation-reduction reaction control means forcontrolling the oxidation-reduction reaction in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be reduced by the sulfurous acid in theslurry to become tetravalent.

In the combustion exhaust gas treatment system of (1), most of Se incombustion exhaust gas is removed by the dust collector in a state beingcontained in the dust, but at least tetravalent Se thereof is, beforebeing removed by the dust collector, made insoluble by reacting with thetreating agent for making at least tetravalent Se insoluble (hereinaftermerely called treating agent) mixed in the circulation liquid of thedesulfurization apparatus sprayed into the combustion exhaust gaslead-in passage by the spraying means. Accordingly, when the other Secontent than tetravalent Se is less, the Se elution standard may besatisfied if the dust after treatment may be directly recycled ordiscarded.

If hexavalent and other Se or other impurities mix into the slurry inthe desulfurization apparatus, most of hexavalent Se reacts withsulfurous acid absorbed from the combustion exhaust gas in the slurry inthe desulfurization apparatus, and is reduced to change to tetravalentSe, and mainly tetravalent Se exists in the circulation liquid in thedesulfurization apparatus. Consequently, this Se and other impuritiesare led into the mixing means as the circulation liquid is extracted,and mixed with the treating agent, and sprayed into the combustionexhaust gas lead-in passage, and most Se is removed by the dustcollector, together with dust, in an insoluble state. If Se and otherimpurities are slightly mixed into the desulfurization apparatus withoutbeing removed by the dust collector, the Se and other impurities areprevented from being accumulated excessively in the circulation liquidof the desulfurization apparatus by the functions of the mixing meansand spraying means, so that the wastewater treating apparatus fortreating the wastewater of the desulfurization apparatus is not needed.

Moreover, by using the desulfurization apparatus having a cooling anddust collecting column upstream of the absorption column, fine dustparticles not captured by the electrostatic precipitator can be capturedin the cooling and dust collecting column, and hardly mix into theslurry of the absorption column, and therefore higher desulfurizationperformance is achieved, and the collected gypsum is higher in quality.

In the combustion exhaust gas treatment system of (2), most Se incombustion exhaust gas is directly fed into the desulfurizationapparatus, together with the combustion exhaust gas, in a state beingcontained in the dust, but at least tetravalent Se thereof is madeinsoluble by reacting with the treating agent mixed in the circulationliquid of the desulfurization apparatus sprayed into the combustionexhaust gas lead-in passage by the spraying means. Accordingly, at leasttetravalent Se contained in the combustion exhaust gas is directlydischarged as being mixed into the solid matter (gypsum, etc.) separatedand formed from the slurry in the desulfurization apparatus.

If hexavalent Se mixed into the desulfurization apparatus, most of thehexavalent Se reacts with sulfurous acid absorbed in the slurry in thedesulfurization apparatus, and is reduced to be tetravalent Se, and alsofinally reacts with the treating agent added by the mixing means to beinsoluble, and is discharged as being mixed in the solid matter (dustcake, etc.) separated and formed in the desulfurization apparatus.

Therefore, in this system, too, the Se elution standard ban be easilysatisfied, and without requiring wastewater treating apparatus, the Seand others are prevented from being accumulated excessively in theabsorption liquid in the desulfurization apparatus.

In the combustion exhaust gas treatment system of (3), most Se incombustion exhaust gas is directly fed into the cooling and dustcollecting column of the desulfurization apparatus, together with thecombustion exhaust gas, in a state being contained in the dust, but atleast tetravalent Se thereof is made insoluble by reacting with thetreating agent mixed in the circulation liquid of the desulfurizationapparatus sprayed into the combustion exhaust gas lead-in passage by thespraying means. Accordingly, at least tetravalent Se contained in thecombustion exhaust gas is directly discharged as being mixed into thesolid matter (gypsum, etc.) separated and formed by the separating meansfor separating the circulation slurry in the cooling and dust collectingcolumn into solid and liquid.

If hexavalent Se mixed into the cooling and dust collecting column ofthe desulfurization apparatus, most of the hexavalent Se reacts withsulfurous acid absorbed in the liquid in the cooling and dust collectingcolumn, and is reduced to be tetravalent Se, and also finally reactswith the treating agent added by the mixing means to be insoluble, andis discharged as being mixed in the solid matter separated and formed bythe separating means.

Therefore, in this system, too, the Se elution standard ban be easilysatisfied. Moreover, without requiring wastewater treating apparatus,the Se and others are prevented from being accumulated excessively inthe circulation liquid in the desulfurization apparatus. Still more, inthis system, since dust does not mix into the slurry in the absorptioncolumn of the desulfurization apparatus, the desulfurization rate in thedesulfurization apparatus, the purity of gypsum, and other performancesmay be kept high.

In the combustion exhaust gas treatment system of (4), theoxidation-reduction reaction control means controls theoxidation-reduction reaction of the slurry in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost completely reduced by thesulfurous acid in the slurry to become tetravalent. Accordingly, ifexisting in the combustion exhaust gas, the hexavalent Se can be changedto tetravalent almost completely in the desulfurization apparatus, andthe Se in combustion exhaust gas can be easily and completely treated.When the desulfurization apparatus comprises cooling and dust collectingcolumn, most of hexavalent Se is removed in the cooling and dustcollecting column, but where the cooling and dust collecting column isnot provided, the installation of this oxidation-reduction reactioncontrol means is particularly effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas passing area of a combustion exhaust gastreatment system in embodiment 1 of the first invention.

FIG. 2 is a schematic explanatory diagram showing an essentialconstitution of the combustion exhaust gas treatment system in FIG. 1.

FIG. 3 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 2 of the firstinvention.

FIG. 4 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 3 of the firstinvention.

FIG. 5 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 4 of the firstinvention.

FIG. 6 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 5 of the firstinvention.

FIG. 7 is a diagram showing an essential constitution of a combustionexhaust gas treatment system in embodiment 1 of the second invention.

FIG. 8 is a diagram showing an essential constitution of a combustionexhaust gas treatment system in embodiment 2 of the second invention.

FIG. 9 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 1 of the third invention.

FIG. 10 is a more specific structural diagram of the constitution of thecombustion exhaust gas treatment system in FIG. 1.

FIG. 11 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 2 of the third invention.

FIG. 12 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 3 of the third invention.

FIG. 13 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 4 of the third invention.

FIG. 14 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 5 of the third invention.

FIG. 15 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 1 of the fourth invention.

FIG. 16 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 2 of the fourth invention.

FIG. 17 is a schematic structural diagram of a combustion exhaust gastreatment system in embodiment 3 of the fourth invention.

FIG. 18 is a graph showing the relation between dust heating temperatureand Se concentration in elution liquid in an elution test in embodiment1 of the fourth invention.

FIG. 19 is a graph showing the relation between dust heating temperatureand Se concentration in elution liquid in an elution test in embodiment1 of the fourth invention.

FIG. 20 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 1 of the fifthinvention.

FIG. 21 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 2 of the fifthinvention.

FIG. 22 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 3 of the fifthinvention.

FIG. 23 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 4 of the fifthinvention.

FIG. 24 is a schematic structural diagram showing an example of aconventional combustion exhaust gas treatment system.

FIG. 25 is a schematic structural diagram showing other example of aconventional combustion exhaust gas treatment system.

FIG. 26 is a schematic structural diagram showing a different example ofa conventional combustion exhaust gas treatment system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A: First Invention:

Referring now to the drawings, embodiments of the first invention aredescribed below.

Embodiment 1

FIG. 1 is a schematic explanatory diagram showing a constitution of acombustion exhaust gas passing area in an example of a combustionexhaust gas treatment system of the first invention, and FIG. 2 is aschematic explanatory diagram showing an essential constitution of thecombustion exhaust gas treatment system. This combustion exhaust gastreatment system comprises, as shown in FIG. 1, a cooler 121 (coolingmeans) for cooling combustion exhaust gas A released from a coal firedboiler 120, an electrostatic precipitator 122 (dust collecting means,sorting means) disposed downstream of the cooler 121 for collecting dustsuch as fly ash in the combustion exhaust gas A and separating from thecombustion exhaust gas, and a fan 123 for feeding the combustion exhaustgas (being rid of dust) discharged from the electrostatic precipitator122 into a later process such as a wet combustion exhaust gasdesulfurization apparatus. At the dust discharge side of theelectrostatic precipitator 122, as shown in FIG. 2, a dust conveyor 124,an Se treating apparatus 125 (Se treating means) for making Seinsoluble, and a solid-liquid separator 126 for separating the dustslurry after Se insoluble treatment into solid and liquid are disposedsequentially.

This system may be incorporated, for example, in the conventionalcombustion exhaust gas treatment system shown in FIGS. 24, 25, 26, andconnected to the desulfurization apparatus for removing sulfurous acidfrom the combustion exhaust gas sent by the fan 123. Of course, theconventional air heater or gas heater can be function as the cooler 121.

The cooler 121 can set the outlet gas temperature, for example, in arange of 150 to 400° C., and the temperature may be set so that thecombustion exhaust gas may be cooled to a temperature sufficient forcondensing the Se in the combustion exhaust gas. More specifically, thetemperature is set so that the combustion exhaust gas may be cooled to350° C. or less, or preferably 310° C. or less. The lower limit of thecooling temperature is not particularly specified, but practically it isabout 90° C.

The electrostatic precipitator 122 has plural hoppers 131 to 134(recovery units) for separating and collecting dust, and these hoppers131 to 134 are formed sequentially from the inlet side (upstream side)to the outlet side (downstream side) of the combustion exhaust gas. Insuch constitution, dust of larger particle size is collected from theinlet side hopper, and dust of smaller particle size is collected fromthe outlet side hopper.

The conveyor 124 is design to convey dust B1 to B4 collected anddischarged from the hoppers 311 to 134 of the electrostatic precipitator122 into one place in batch (Se treating apparatus 125).

The Se treating apparatus 125 has a function of adding water or otherliquid to the dust to form into slurry, and adding a treating agent Cthereto to mix, and, for example, the repulping tank for forming slurryand the mixing tank for adding and mixing the treating agent C may beseparately disposed, or these functions may be realized by a singletank.

As the treating agent C, a chemical reacting with SE to make itinsoluble is necessary, and if there is at least tetravalent Se (mainform: selenious acid SeO₃ ²⁻) among Se components to be removed, forexample, FeCl₃ or Fe₂ (SO₄)₃ may be used.

If there is hexavalent Se (main form: selenic acid SeO₄ ²⁻) in thecombustion exhaust gas and it is needed to make it insoluble in order toconform to the elution standard, as the treating agent, a reducing agentfor transforming hexavalent Se into tetravalent Se (e.g. Na₂SO₃) and theabove chemical may be charged. In this case, a pretreatment tank forreduction reaction of hexavalent Se into tetravalent Se may be providedaside from the mixing tank as the constitution of the Se treatingapparatus 125.

In this Se treating apparatus 125, if it is necessary to repulp thedust, the circulation water of the desulfurization apparatus may be usedas the solvent.

The charging amount of the treating agent C may be set slightly morethan the stoichiometric equivalent for making Se in the dust completelyinsoluble determined from the reaction mentioned below (reactionformulas 1, 2, or 3, 4).

In thus constituted combustion exhaust gas treatment system, dustremoval treatment and harmless treatment of Se contained much in thedust are performed as follows.

That is, the combustion exhaust gas A leaving the boiler 120 is firstcooled by the cooler 121 to 350° C. or less, and therefore at least atthe downstream side of the cooler 121, the Se in the combustion exhaustgas is condensed and the majority deposits on the ash which composes thedust. This dust is separated and collected by the electrostaticprecipitator 122, and all the separated and collected dust is fed intothe Se treating apparatus 125 in batch by the conveyor 124.

In the Se treating apparatus 125, the tetravalent Se (main form:selenious acid SeO₃ ²⁻) contained in the slurry formed by repulsing ofthe supplied dust (hereinafter called dust slurry) reacts with thetreating agent (FeCl₃ or Fe₂(SO₄)₃) as shown in the following reactionformulas 1, 2, or 3, 4, and is made insoluble in a form of iron selenite(Fe₂(SeO₃)₃).

When hexavalent Se (main form: selenic acid SeO₄ ²⁻) is present, thereducing agent as mentioned above is charged, and this hexavalent Sereacts with the reducing agent to be tetravalent Se, which similarlyreacts as shown in the following reaction formulas 1, 2, or 3, 4 to beinsoluble.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe⁺3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ³⁻→Fe₂(SeO₃)₃↓(4)

Accordingly, when the dust slurry making Se insoluble is separated intosolid and liquid in the solid-liquid separator 126, most of Se isseparated to the solid-phase side as iron selenite, and is mixed ininsoluble form into the dust cake D discharged from the solid-liquidseparator 126. Therefore, if the dust cake D is directly discarded inthe ash disposal yard, it conforms to the elution standard.Incidentally, since the filtrate E from the solid-liquid separator 126is extremely low in Se concentration, it can be easily treated aswastewater. Or it may be used as addition water for dust slurry, orreturned to the desulfurization apparatus as circulation water forabsorbent slurry of the desulfurization apparatus.

The result of combustion exhaust gas treatment experiment by the testapparatus constructed as shown in FIGS. 1 and 2 is explained below. Inthe experiment, coal containing 3 mg/kg of Se was supplied into acombustion furnace at a rate of 25 kg/h, and combustion exhaust gas of200 m³ N/h discharged from the combustion furnace was cooled to 150° C.,and fed into an electrostatic precipitator. As a result, more than 99%of dust was captured by the electrostatic precipitator (Se capturingrate about 99.4%), and the amount of dust collected by the conveyor (thetotal collected from the hoppers) was 3.4 kg/h. As a result of analysisof the Se concentration in the dust (B1 to B4) collected by the conveyoraccording to the elution test and atomic absorption method by hydrogencompound generating method conforming to the ordinance No. 13 ofEnvironmental Agency of Japan, the concentration (Se componentscontained in the dust and eluting by elution test) was 0.33 mg/liter,which was over the landfill elution standard (0.3 mg/liter). However, in30 minutes after mixing by adding treating agent C (using FeCl₃) in theSe treating apparatus 125, by separating into solid and liquid by thesolid-liquid separator 126, the Se concentration was measured in thesolid-phase side (dust cake D) and liquid phase side (filtrate E), andthe results are shown in Table 1, which sufficiently satisfied theelution standard (0.3 mg/liter). That is, the eluting Se concentrationin the solid-phase side (Se components contained in the solid phase andeluting by the elution test) was 0.04 to 0.1 mg/liter, and the elutingSe concentration in the liquid-phase side was all 0.01 mg/liter or less.

TABLE 1 Eluting selenium concentration from solid phase after SeleniumSlurry FeCl₃ solid-liquid concentration in concentration concentrationseparation liquid phase after (wt. %) (wt. %) (mg/liter) (mg/liter) 250.1 0.10 25  0.25 0.08 25 0.5 0.07 25 1.0 0.05 25 2.5 0.04 All 0.01 orless 25 5.0 0.04 50 0.5 0.08 50 1.0 0.06 50 5.0 0.05

As described herein, according to the combustion exhaust gas treatmentsystem of the embodiment, the Se in the combustion exhaust gas isremoved together with the dust, without requiring the complicatedaftertreatment needed in the prior art, and is finally contained in thedust cake D in insoluble form, so as to be discarded directly. In thiscombustion exhaust gas treatment system, only by adding the Se treatingapparatus 125 and solid-liquid separator 126 to the conventionalcombustion exhaust gas treatment system shown in FIGS. 24 to 26, thecombustion exhaust gas treatment system incorporating the system of theinvention can be realized, and the Se in the combustion exhaust gas ismade harmless, and modification of the existing combustion exhaust gastreatment system is easy, and when newly installing this system, theconventional design or equipment may be used as it is.

Embodiment 2

FIG. 3 is a schematic explanatory diagram showing a constitution of asecond example of a combustion exhaust gas treatment system of the firstinvention. Same constituent elements as in embodiment 1 are identifiedwith same reference numerals, and their explanations are omitted. Thiscombustion exhaust gas treatment system possesses, as shown in FIG. 3, aconveyor 141 for feeding only dust B3, B4 separated and collected fromspecific hoppers 133, 134 at the outlet side of the combustion exhaustgas, out of plural hoppers 131 to 134 in the electrostatic precipitator122, into the Se treating apparatus 125, and a conveyor 142 forconveying and treating the dust B1, B2 separated and collected fromspecific hoppers 131, 132 at the inlet side of the combustion exhaustgas, separately from the dust B3, B4, and is characterized by making Seinsoluble only in part of the dust B3, B4, and discarding the remainingdust B1, B2. The electrostatic precipitator 122 in this embodimentfunction as t he dust collecting means and the sorting means in thefirst invention.

In this case, only the dust B3, B4 separated and collected from thespecific hoppers 133, 134 at the outlet side of the combustion exhaustgas undergoes Se insoluble treatment, and therefore the required amountof the treating agent C and the required capacity of the Se treatingcapacity 125 can be decreased, and the Se is made harmless more easilyand inexpensively.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash) separated and collected from the specific recovery unit at theoutlet side, and the Se is made harmless on the whole only by applyinginsoluble treatment on the dust of smaller particle size, therebycontributing to reduction of facility cost and running cost.

The result of combustion exhaust gas treatment experiment by the testapparatus in FIG. 3 is described below. In the experiment, coalcontaining 3 mg/kg of Se was supplied into a combustion furnace at 4rate of 25 kg/h, and the flue of 200 m³ N/h released from the combustionfurnace was cooled to 150° C., and fed into an electrostaticprecipitator. In this case, more than 99% of the dust was captured bythe electrostatic precipitator (capturing rate of Se about 99.4%), andthe amount of dust collected by the conveyors (the total collected fromthe hoppers) was 3.4 kg/h. The discharge amount of dusts (collected ash)conveyed and collected by the conveyors 141, 142, the mean particlesize, and eluting Se concentration (Se components contained in thecollected ash and eluting by the elution test) were as shown in Table 2.

TABLE 2 Collected ash Collected ash discharged discharged from conveyor142 from conveyor 141 (combustion exhaust (combustion exhaust Collectedash gas gas item inlet side) outlet side) Discharge amount kg/h 2.271.14 Mean particle size of collected 12 5 ash μm Eluting selenium 0.200.49 concentration in collected ash mg/liter

More specifically, the discharge amount of the dust B3, B4 separated andcollected from the hoppers 133, 134 at the outlet side of the combustionexhaust gas was slight, 1.14 kg/h, but the eluting Se concentration was0.49 mg/liter, high above the standard. On the other hand, the dischargeamount of the dust B1, B2 separated and collected from the hoppers 131,132 at the inlet side of combustion exhaust gas was large, 2.27 kg/h,but the eluting Se concentration was 0.20 mg/liter, far below thestandard. Accordingly, it is known that the dust B1, B2 separated andcollected from the hoppers 131, 132 at the inlet side of the combustionexhaust gas can be directly discarded. That is, Se insoluble treatmentis not needed in the dust at the inlet side of the combustion exhaustgas which is about twice larger in the discharge amount, and hence it isevident that the required amount of the treating agent C and therequired capacity of the Se treating apparatus 125 can be substantiallysaved.

Incidentally, such difference in eluting Se concentration is regarded tobe due to the particle size of dust (ash). That is, when gaseous Se(SeO₂) is condensed and deposits on the surface of the ash forming thedust, ash of smaller particle size is greater in the specific surfacearea per unit weight, and hence more Se deposits. On the other hand, inthe dust collector such as the electrostatic precipitator mentionedabove, coarse ash particles are likely to be captured at the inlet sideof the combustion exhaust gas, and fine ash particles are likely to becaptured at the outlet side of the combustion exhaust gas, and in otherwords there is a sorting function, and it is hence considered that theeluting Se concentration is high in the dust captured at the outlet sideof the combustion exhaust gas.

The dust B3, B4 separated and collected from the hoppers 133, 134 at theoutlet side of the combustion exhaust gas were fed into the Se treatingapparatus 125 by the conveyor 141, and the treating agent C (usingFeCl₃) was added and mixed, and 30 minutes later, it was separated intosolid and liquid in the solid-liquid separator 126, and the Seconcentration was measured in the solid-phase side (dust cake D) andliquid-phase side (filtrate E), and the measurements as shown in Table 3were obtained, which sufficiently satisfied the elution standard (0.3mg/liter). That is, the eluting Se concentration in the solid-phase side(Se components contained in the solid phase and eluting by the elutiontest) was 0.05 to 0.09 mg/liter, and the eluting Se concentration in theliquid-phase side was 0.01 mg/liter or less. In embodiments 1 and 2, atthe same slurry concentration and treating agent (FeCl₃) concentration,the consumption was the treating agent is compared in Table 4, whichshows that it is decreased to about ⅓ in embodiment 2 as compared withembodiment 1.

TABLE 3 Eluting selenium Selenium concentration concentration in fromsolid liquid phase after Slurry FeCl₃ phase after solid-liquidconcentration concentration solid-liquid separation (wt. %) (wt. %)separation (mg/liter) (mg/liter) 25 0.5 0.08 25 1.0 0.06 25 2.5 0.05 500.5 0.09 All 0.01 or less 50 1.0 0.07 50 5.0 0.05

TABLE 4 Embodiment 1 Embodiment 2 Dust amount (kg/h) 3.4 1.14 Slurryconcentration (wt. %) 25 25 FeCl₃ concentration (wt. %) 0.5 0.5Consumption of treating agent (kg/h) 0.051 0.017

Embodiment 3

FIG. 4 is a schematic explanatory diagram showing a constitution of athird example of a combustion exhaust gas treatment system of the firstinvention. Same constituent elements as in embodiment 1 are identifiedwith same reference numerals and their explanations are omitted. Thiscombustion exhaust gas treatment system, as shown in FIG. 4, comprises asorter 151 (sorting means) for classifying the dust B1 to B4 captured bythe electrostatic precipitator 122 and conveyed in batch by the conveyor124 into large particle size (coarse ash) B5 and small particle size(fine ash) B6, and only fine ash B6 sorted by the sorter 151 is capturedby a fine particle capturing apparatus 152, and fed into the Se treatingapparatus 125 to make Se insoluble.

Meanwhile, in this case, the dust B1 to B4 conveyed in batch by theconveyor 124 are conveyed by air F and led into the sorter 151. Thesorter 151 may be constituted by, for example, a cyclone, and it isconvenient when it is designed to adjust the degree of sorting. As thefine particle capturing apparatus 152, in this case, a bag filter isused.

In this case, only the fine ash B6 is subjected to Se insolubletreatment, and same as in embodiment 2, therefore, the required amountof treating agent C and required capacity of Se treating apparatus 125can be reduced, so that the Se may be made harmless more easily andinexpensively.

In this combustion exhaust gas treatment system, only by adding the Setreating apparatus 125 and sorter 151 to the conventional combustionexhaust gas treatment system shown in FIGS. 24 to 26, the combustionexhaust gas treatment system incorporating the system of the inventioncan be realized, while the conveyor and other structures may be thesame, and the Se in the combustion exhaust gas is made harmless, andmodification of the existing combustion exhaust gas treatment system iseasy, and when newly installing this system, the conventional design orequipment may be used as it is.

The result of combustion exhaust gas treatment experiment by the testapparatus shown in FIG. 4 is described below. In the experiment, coalcontaining 3 mg/kg of Se was supplied into a combustion furnace at arate of 25 kg/h, and the combustion exhaust gas exhausted from thecombustion furnace at a rate of 200 m³ N/h was cooled to 150° C. and fedinto the electrostatic precipitator. In this case, more than 99% of thedust was captured by the electrostatic precipitator, and the amount ofdust collected by the conveyor (the total collected from the hoppers)was 3.4 kg/h. The capturing amount of coarse ash B5 and fine ash B6,mean particle size, and eluting Se concentration (Se componentscontained in captured ash and eluting by the elution test) are shown inTable 5.

TABLE 5 Collected ash Item Coarse ash B5 Fine ash B6 Capturing amountkg/h 2.05 1.30 Mean particle size of captured ash 13 5.4 μm Elutingselenium concentration in 0.26 0.36 captured ash mg/liter

That is, in the fine ash B6, the mean particle size was 5.4 μm, and thecapturing amount was small, 1.30 kg/h, but the eluting Se concentrationexceeded the standard, 0.36 mg/liter. In coarse ash B5, the meanparticle size was 13 μm, the capturing amount was large, 2.05 kg/h, butthe eluting Se concentration was below the standard, 0.26 mg/liter.Accordingly, it is known that the coarse ash B5 can be directlydiscarded. Hence, Se insoluble treatment is not needed in the coarse ashB5 which is very large in output, and it is evident that the requiredamount of treating agent C and required capacity of Se treatingapparatus 125 can be saved substantially.

The fine ash B6 was fed into the Se treating apparatus 125, and blendedwith the treating agent C and mixed for 30 minutes, and it was separatedinto solid and liquid by the solid-liquid separator 126, and the Seconcentration was measured in the solid-phase side (dust cake D) andliquid-phase side (filtrate E), and the same results as in embodiment 2shown in Table 3 were obtained, and the elution standard (0.3 mg/liter)was sufficiently satisfied.

Embodiment 4

FIG. 5 is a schematic explanatory diagram showing a constitution of afourth example of a combustion exhaust gas treatment system of the firstinvention. Same constituent elements as in embodiment 2 in FIG. 3 areidentified with same reference numerals and their explanations areomitted. This combustion exhaust gas treatment system is, as shown inFIG. 5, characterized by comprising a mixer 161 (mixing means) formixing the dust G undergoing Se insoluble treatment as being fed intothe Se treating apparatus 125 from the conveyor 141 after beingseparated and collected by the hoppers 133, 134 at the outlet side ofthe combustion exhaust gas, and the other dust B1, B2 not undergoing Seinsoluble treating after being separated and collected by the hoppers131, 132 at the inlet side of the combustion exhaust gas, so that thewater content may be 20% or less (preferably 10% or less).

The Se treating means is an Se treating apparatus 125 a capable ofinducing Se insoluble reaction (for example, reaction formulas 1, 2, or3, 4), by spraying a liquid containing a treating agent such as FeCl₃ orFe₂ (SO₄)₃ (concentration about 0.5 to 5.0 wt. %) uniformly into thedust.

In this example, the dust G undergoing Se insoluble treatment, and theother dust B1, B2 not undergoing Se insoluble treatment are mixed by themixer 161, and a harmless dust cake H (mixed ash) of low water contentis. formed. Accordingly, without using large-scale equipment such assolid-liquid separator that requires wastewater (filtrate) treatment,the water content in the dust can be easily lowered, and the harmlessdust can be handled easily in waste disposal. In the constitution of theembodiment, the water content of the mixed ash H and eluting Seconcentration (Se components contained in mixed ash and eluting by theelution test) were measured in the same conditions as in embodiment 2,of which results are shown in Table 6.

TABLE 6 FeCl₃ Spray flow Water concentration rate of content in Elutingselenium in spray liquid FeCl₃ solution mixed ash concentration in mixed(wt. %) (g/h) (wt. %) ash (mg/liter) 0.5 400  9.7 0.18 1.0 400 10.5 0.152.5 400 10.6 0.14 5.0 400  9.8 0.16 5.0 600 15.4 0.13 5.0 800 19.9 0.11

Embodiment 5

FIG. 6 is a schematic explanatory diagram showing a constitution of afifth example of a combustion exhaust gas treatment system of the firstinvention. Same constituent elements as in embodiment 4 in FIG. 5 areidentified with same reference numerals and their explanations areomitted. This combustion exhaust gas treatment system is, as shown inFIG. 6, characterized by comprising a scale forming apparatus 171 (scaleforming means) for compacting the dust H mixed by the mixer 161 to forminto scale.

In this case, the scale forming apparatus 171 forms the dust H intoscale dust (scale ash) I, and the volume of the dust H is furtherreduced, and handling in waste disposal is much easier. Besides, thewater content of the dust H is kept under 20% (preferably under 10%) bythe mixer 161, which is effective for making it easier to operate tocompact by the scale forming apparatus 171. As a result of experiment inthe same running conditions as in embodiment 4, the bulk density of thedust cake H (mixed ash) mixed by the mixer 161 was 0.8 g/cc, whereas thebulk density of the dust (scale ash) I formed into scale by the scaleforming apparatus 171 was 1.5 g/cc.

The first invention may be realized also in other various forms than theillustrated embodiments. For example, as the treating agent for makingSe insoluble, aside from FeCl₃ or Fe₂ (SO₄)₃, chelating agents (e.g.Miyoshi Resin Epolus MX-7), and high polymer heavy metal capturingagents (e.g. Miyoshi Resin Epofloc L-1) can be used. The dust collectingmeans and sorting means of the invention are not limited to theelectrostatic precipitator and single cyclone connected thereto, but thedust collecting means and sorting means of the invention may berealized, for example, by multiple cyclones.

The combustion exhaust gas treatment system of the first invention maybe incorporated in part of the combustion exhaust gas treatment systemhaving a conventional desulfurization apparatus. For example, inembodiment 1, the desulfurization apparatus for removing sulfurous acidfrom the combustion exhaust gas sent by the fan 123 may be provided, forexample, as in the prior art shown in FIGS. 24 to 26. Besides, theconventional air heater or gas-gas heater may be used as the coolingmeans of the first invention.

Effects of the First Invention

According to the combustion exhaust gas treatment system of the firstinvention, most of Se in combustion exhaust gas is cooled by the coolingmeans and condensed, and is removed by the dust collecting means in astate being contained in the dust. To the dust separated by the dustcollecting means, a treating agent is added by the Se treating means,and the existent form of Se in the dust is transformed into an insolublecompound. Accordingly, if the dust is discarded same as in the priorart, the Se elution standard is satisfied, and Se is made harmlesseasily without requiring complicated aftertreatment.

By applying Se insoluble treatment only on the dust separated andcollected from the specific recovery unit at the outlet side ofcombustion exhaust gas in the dust collecting means, the required amountof treating agent and required capacity of the Se treating means can bereduced, and the Se is made harmless more easily and economically.

Also by applying Se insoluble treatment only on the dust of smallparticle size sorted by the sorting means, the required amount oftreating agent and required capacity of the Se treating means can bereduced, and the Se is made harmless more easily and economically.

By installing mixing means, when the dust undergoing Se insolubletreatment and remaining dust not undergoing Se insoluble treatment aremixed so that the water content may be 20% or less, the water content ofthe dusts may be easily lowered without using large-scale equipment suchas solid-liquid separator that requires treatment of wastewater(filtrate), and therefore handling in disposal of dust may be easier.

By installing scale forming means, when the dust mixed by the mixingmeans is formed into scale, handling in disposal of dusts may be mucheasier.

B: Second Invention

Referring now to drawings, embodiments of the second invention aredescribed below.

Embodiment 1

FIG. 7 is a diagram showing an essential structure of a combustionexhaust gas treatment system in embodiment 1 of the second invention.This combustion exhaust gas treatment system comprises, as shown in FIG.7, a cooler 221 (cooling means) for cooling combustion exhaust gas Aexhausted from a coal fired boiler 220, an electrostatic precipitator222 (dust collecting means, sorting means) disposed downstream of thecooler 221, for collecting dust such as fly ash in the combustionexhaust gas A, and separating from the combustion exhaust gas, and a fan223 for supplying the combustion exhaust gas (being rid of dust)discharged from the electrostatic precipitator 222 to a later process.

At the dust discharge side of the electrostatic precipitator 222, thereare, as sequentially arranged, a mixer 225 (mixing means) for adding andmixing Se elution preventive agent C and humidifying liquid D to thedust, a briquetting machine 226 (scale forming means) for forming thedust discharged from the mixer 225 into scale, a granulator 227(granulating means) for granulating the scale formed dust (scale dust)formed by the briquetting machine 226 into a size suited to handling,and a screen 228 (sorting means) for screening a proper size of thescale dust granulated by the granulator 227.

Moreover, a desulfurization apparatus for removing sulfurous acid fromthe combustion exhaust gas sent by the fan 223 may be provided same asin the prior art shown in FIGS. 24 to 26. Besides, as a matter ofcourse, the conventional air heater or gas-gas heater may be function asthe cooler 221.

Of the scale dust sorted by the screen 228, scale dust E of proper sizemay be directly discarded or recycled, and smaller scale dust F is putback into the mixer 225, and larger scale dust G is sent again into thegranulator 227.

The cooler 221 is to set the outlet gas temperature, for example, in arange of 150 to 400° C., and the temperature may be set so that thecombustion exhaust gas may be cooled to the temperature for sufficientlycondensing the Se in the combustion exhaust gas. Specifically, thetemperature is set so as to cool the combustion exhaust gas to 350° C.or less, or preferably 310° C. or less.

The electrostatic precipitator 222 has plural hoppers 231 to 234(recovery units) for separating and collecting dust, and these hoppers231 to 234 are formed sequentially from the inlet side (upstream side)to the outlet side (downstream side) of the combustion exhaust gas, andin such constitution, dust of larger particle size is collected from theinlet side hopper, and dust of smaller particle size is collected fromthe outlet side hopper.

In this case, only the dust B3, B4 separated and collected from thespecific hoppers 233, 234 at the outlet side of the combustion exhaustgas, out of the plural hoppers 231 to 234 in the electrostaticprecipitator 222, are led into the mixer 225 to make Se insoluble, whilethe remaining dust B1, B2 are discarded directly.

The mixer 225 has the function of, in this case, mixing and dischargingthe charged dust, Se elution preventive agent C, and humidifying liquidD, and is provided with, for example, agitation blades inside forsending out the charged matter to the discharge side while agitating it.

Herein, as the Se elution preventive agent, a chemical reacting with Seto make it insoluble is necessary, and for example, FeCl₃ or Fe₂ (SO₄)₃may be used if at least tetravalent Se (main form: selenious acid SeO₃²⁻) is contained among Se components to be removed.

If there is hexavalent Se (main form: selenic acid SeO₄ ²⁻) in thecombustion exhaust gas and it must be made insoluble in order to conformto the elution standard, a reducing agent for transforming hexavalent Seinto tetravalent Se may be added as treating agent, together with theabove chemical. As the reducing agent, for example, sulfurous acid waterobtained by blowing SO₂ into water may be preferably used. When the wetdesulfurization apparatus is installed at the same time, it is preferredto absorb SO₂ by the desulfurization apparatus, and extract and use theslurry or circulation water containing unreacted sulfurous acid.

The charging amount of the Se elution preventive agent C may be setslightly more than the stoichiometric equivalent for making Se in thedust completely insoluble determined from the reaction formulas 1, 2, or3, 4.

As the humidifying liquid D, aside from ordinary industrial water, theslurry or circulation water of the desulfurization apparatus may beused, and its charging amount may be a minimum required level for theease of handling of dust or for compacting and forming into scale. Forexample, when the dust amount is 15 t/h, FeCl₃ may be charged by about150 kg/h, and humidifying water about 0.79 t/h. Herein, as thehumidifying liquid, by absorbing SO₂ in the desulfurization apparatus,the slurry or circulation water containing unreacted sulfurous acid maybe used, and the dissolved sulfurous acid may act as reducing agent forreducing the hexavalent Se into tetravalent Se.

Instead of adding the Se elution preventive agent C by charging togetherwith the humidifying liquid D, it may be preliminarily mixed in asolution and added. In such a case, the solution concentration ispreferred to be about 0.05 to 5 wt. %, and the loading may be about 0.5to 10 wt. % to the dust. By so setting, most of Se is made insoluble,and the dust may be handled easily without discharge of filtrate, and itmay be smoothly compacted and formed into scale.

In thus constituted combustion exhaust gas treatment system, dustremoval treatment and harmless treatment of Se much contained in dustare executed in the following procedure.

The combustion exhaust gas A released from the boiler 220 is firstcooled to 350° C. or less by the cooler 221, and therefore at least atthe downstream side of the cooler 221, the Se in combustion exhaust gasis condensed, and mostly deposits on the ash which forms the dust in thecombustion exhaust gas. This dust is separated and collected by theelectrostatic precipitator 222, and the dust B1, B2 separated andcollected from the specific hoppers 231, 232 at the inlet side of thecombustion exhaust gas are directly discarded, while only the dust B3,B4 separated and collected from the specific hoppers 233, 234 at theoutlet side of the combustion exhaust gas are fed into the mixer 225.

In the mixer 225 or subsequent briquetting machine 226, the tetravalentSe (main form: selenious acid SeO₃ ²⁻) contained in the charged dustreacts with the Se elution preventive agent (FeCl₃ or Fe₂ (SO₄)₃) asshown in the following reaction formulas 1, 2, or 3, 4, and is madeinsoluble in a form of iron selenite (Fe₂ (SeO₃)₃).

When hexavalent Se (main form: selenic acid SeO₄ ²⁻) is present, thereducing agent as mentioned above is charged, and this hexavalent Sereacts with the reducing agent to be tetravalent Se, which similarlyreacts as shown in the following reaction formulas 1, 2, or 3, 4 to beinsoluble.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ³⁻→Fe₂(SeO₃)₃↓(4)

Accordingly, most of Se is made insoluble and mixed in the scale dust Eas iron selenite. Therefore, if the scale dust E may be directlydiscarded in the ash disposal yard, the elution standard can besatisfied. Besides, the dusts B1, B2 separated and collected from thespecific hoppers 231, 232 of the electrostatic precipitator 222 are lowin Se concentration, and if directly discharged in the ash disposalyard, the elution standard is satisfied.

As described herein, according to the combustion exhaust gas treatmentsystem of embodiment 1, without requiring complicated aftertreatment ofthe prior art, the Se in combustion exhaust gas can be removed togetherwith dust, and finally most of it is made insoluble and present in thescale dust E so as to be discarded directly. Still more, because of theconstitution for adding a humidifying liquid to mix the dust and Seelution preventive agent, and forming into scale, as compared with theconstitution for forming the dust into slurry, mixing insoluble treatingagent, and separating into solid and liquid to discard, it does notrequire large-scale equipment or facility such as wastewater (filtrate)treatment apparatus and solid-liquid separator, and handling of dust inwaste disposal is much easier. If not particularly needed in wastedisposal, the scale forming-process may be omitted, so that the systemmay be further simplified.

In this case, moreover, only the dust B3, B4 separated and collectedfrom the specific hoppers 233, 234 at the outlet side of the combustionexhaust gas are presented to Se insoluble treatment, and therefore therequired amount of Se elution preventive agent C, and required capacityof the mixer 225 (mixing means) and briquetting machine 226 (scaleforming means) can be reduced, and the Se is made harmless more easilyand inexpensively. That is, as mentioned above, since more Se iscontained (deposited) on the dust (ash) of smaller particle sizeseparated and collected from the specific recovery unit at the outletside, only by applying the insoluble treatment on the dust of smallerparticle size, the Se is made harmless on the whole, therebycontributing to reduction of facility cost and running cost.

The result of dust collecting experiment by using the electrostaticprecipitator in the system of FIG. 7 is described below. In theexperiment, coal containing 3 mg/kg of Se was supplied in a combustionfurnace at a rate of 25 kg/h, and combustion exhaust gas exhausted fromthe combustion furnace at a rate of 200 m³ N/h was cooled to 150° C.,and fed into the electrostatic precipitator. In this case, more than 99%of the dust was captured by the electrostatic precipitator (Se capturingrate about 99.4%), and the total amount of dust collected from thehoppers was 3.4 kg/h. The discharge amount of dust (collected ash)collected from the hoppers 231, 232, or 233, 234, the mean particlesize, and eluting Se concentration are shown in Table 7. Herein, theeluting Se concentration refers to the concentration of Se in the dustanalyzed according to the elution test and atomic absorption methodconforming to the ordinance No. 13 of Environmental Agency of Japan, andit shows the amount of Se contained in the dust and eluting in theelution test.

TABLE 7 Collected ash Collected ash discharged discharged from hoppersfrom hoppers 231, 232 233, 234 (combustion (combustion Collected ashexhaust gas exhaust gas item inlet side) outlet side) Discharge amount(kg/h) 2.27 1.14 Mean particle size of collected ash 12 5 (μm) Seleniumeluting concentration in 0.20 0.49 collected ash (mg/liter)

More specifically, the discharge amount of the dust B3, B4 separated andcollected from the hoppers 233, 234 at the outlet side of the combustionexhaust gas was slight, 1.14 kg/h, but the eluting Se concentration was0.49 mg/liter, high above the landfill standard (0.3 mg/liter). On theother hand, the discharge amount of the dust B1, B2 separated andcollected from the hoppers 231, 232 at the inlet side of combustionexhaust gas was large, 2.27 kg/h, but the eluting Se concentration was0.20 mg/liter, far below the standard. Accordingly, it is known that thedust B1, B2 separated and collected from the hoppers 231, 232 at theinlet side of the combustion exhaust gas can be directly discarded. Thatis, Se insoluble treatment is not needed in the dust at the inlet sideof the combustion exhaust gas which is about twice larger in thedischarge amount, and hence it is evident that the required amount ofthe Se elution preventive agent C and the required capacity of the mixer225 can be substantially saved.

Incidentally, such difference in eluting Se concentration is regarded tobe due to the particle size of dust (ash). That is, when gaseous Se(SeO₂) is condensed and deposits on the surface of the ash forming thedust, ash of smaller particle size is greater in the specific surfacearea per unit weight, and hence more Se deposits. On the other hand, inthe dust collector such as the electrostatic precipitator mentionedabove, coarse ash particles are likely to be captured at the inlet sideof the combustion exhaust gas, and fine ash particles are likely to becaptured at the outlet side of the combustion exhaust gas, and in otherwords there is a sorting function, and it is hence considered that theeluting Se concentration is high in the dust captured at the outlet sideof the combustion exhaust gas.

Embodiment 2

As other embodiment of the second invention, embodiment 2 is describedbelow. Same constituent elements as in embodiment 1 are identified withsame reference numerals and their explanations are omitted.

FIG. 8 shows an essential structure of a combustion exhaust gastreatment system of embodiment 2. The combustion exhaust gas treatmentsystem of this embodiment comprises a sorter 251 (sorting means) forclassifying the dust B1 to B4 captured by the electrostatic precipitator222 and conveyed in batch into large particle size (coarse ash) B5 andsmall particle size (fine ash) B6, and only fine ash B6 sorted by thesorter 251 is captured by a fine particle capturing apparatus 252, andfed into a mixer 225 to make Se insoluble. In this case, the dust B1 toB4 conveyed in batch are conveyed by air H and led into the sorter 251.The sorter 251 may be constituted by, for example, a cyclone, and it isconvenient when it is designed to adjust the degree of sorting. As thefine particle capturing apparatus 252, in this case, a bag filter isused.

In this case, only the fine ash B6 is subjected to Se insolubletreatment, and same as in embodiment 1, therefore, the required amountof Se elution preventive agent C and required capacity of mixer 225 canbe reduced, so that the Se may be made harmless more easily andinexpensively.

In this combustion exhaust gas treatment system, only by adding themixer 225 and sorter 251 to the conventional combustion exhaust gastreatment system shown in FIGS. 24 to 26, the system can be realized,while the conveyor and other structures for conveying the dust capturedby the electrostatic capacitor 222 may be the same, and the Se in thecombustion exhaust gas is made harmless, and modification of theexisting combustion exhaust gas treatment system is easy, and when newlyinstalling this system, the conventional design or equipment may be usedas it is.

The result of dust collecting experiment by the electrostaticprecipitator in the structure of the system in FIG. 8 is describedbelow. In the experiment, coal containing 3 mg/kg of Se was suppliedinto a combustion furnace at a rate of 25 kg/h, and the combustionexhaust gas exhausted from the combustion furnace at a rate of 200 m³N/h was cooled to 150° C. and fed into the electrostatic precipitator.In this case, more than 99% of the dust was captured by theelectrostatic precipitator, and the total amount of dust collected fromthe hoppers was 3.4 kg/h. The capturing amount of coarse ash B5 and fineash B6, mean particle size, and eluting Se concentration are shown inTable 8.

TABLE 8 Collected ash Item Coarse ash B5 Fire ash B6 Capturing amountkg/h 2.05 1.30 Mean particle size of captured ash 13 5.4 μm Elutingselenium concentration in 0.26 0.36 captured ash mg/liter

That is, in the fine ash B6, the mean particle size was 5.4 μm, and thecapturing amount was small, 1.30 kg/h, but the eluting Se concentrationexceeded the standard, 0.36 mg/liter. In coarse ash B5, the meanparticle size was 13 μm, the capturing amount was large, 2.05 kg/h, butthe eluting Se concentration was below the standard, 0.26 mg/liter.Accordingly, it is known that the coarse ash B5 can be directlydiscarded. Hence, Se insoluble treatment is not needed in the coarse ashB5 which is very large in output, and it is evident that the requiredamount of Se elution preventive agent C and required capacity of themixer 225 can be saved substantially.

The second invention may be realized also in other various forms thanthe illustrated embodiments. For example, as the Se elution preventiveagent, aside from FeCl₃ or Fe₂ (SO₄)₃, chelating agents (e.g. MiyoshiResin Epolus MX-7), and high polymer heavy metal capturing agents (e.g.Miyoshi Resin Epofloc L-1) can be used. The dust collecting means andsorting means of the invention are not limited to the electrostaticprecipitator and single cyclone connected thereto, but the dustcollecting means and sorting means of the invention may be realized, forexample, by multiple cyclones.

The combustion exhaust gas treatment system of the second invention maybe incorporated in part of the combustion exhaust gas treatment systemhaving a conventional desulfurization apparatus. For example, inembodiment 1, the desulfurization apparatus for removing sulfurous acidfrom the combustion exhaust gas sent by the fan 223 may be provided, forexample, as in the prior art shown in FIGS. 24 to 26. Besides, theconventional air heater or gas-gas heater may be used as the coolingmeans of the second invention.

Moreover, all of the dust separated and collected by the electrostaticprecipitator 222 may be treated by introducing into the mixer 225(mixing means).

Effects of the Second Invention

According to the combustion exhaust gas treatment system of the secondinvention, most of Se in combustion exhaust gas is cooled by the coolingmeans and condensed, and is removed by the dust collecting means in astate being contained in the dust. To the dust separated by the dustcollecting means, Se elution preventive agent and humidifying liquid ora solution of Se elution preventive agent is added by the mixing means,and the existent form of Se in the dust is transformed into an insolublecompound. Accordingly, if the dust is discarded same as in the priorart, the Se elution standard is satisfied, and Se is made harmlesseasily without requiring complicated aftertreatment. Moreover, by addingthe humidifying liquid and Se elution preventive agent, or spraying asolution of Se elution preventive agent to mix with the dust, andforming the dust into scale, as compared with the case of forming thedust into slurry, mixing Se insoluble treating agent, and separatinginto solid and liquid to discard, it does not require large scaleequipment or facility such as wastewater (filtrate) treating facility orsolid-liquid separator, and handling is much easier in waste disposal ofdust.

Also by feeding only the dust separated and collected from the specificrecovery unit at the outlet side of the combustion exhaust gas in thedust collecting means or the dust of small particle size sorted by thesorting means into the mixing means to make insoluble, the requiredamount of Se elution preventive agent and required capacity of themixing means and scale forming means can be reduced, and the Se is madeharmless more easily and economically.

C: Third Invention

Embodiments of the third invention is described below while referring tothe accompanying drawings.

Embodiment 1

FIG. 9 is a schematic structural (principle) diagram showing an exampleof a combustion exhaust gas treatment system of the third invention (1)to (3), (6) and (9), and FIG. 10 is a structural diagram specificallyshowing the constitution of the combustion exhaust gas treatment system.In the following explanation, the combustion exhaust gas to be treatedis supposed to contain both hexavalent Se and tetravalent Se.

In the combustion exhaust gas treatment system in the embodiment, asshown in FIG. 9, dust containing Se is removed from the combustionexhaust gas 310 by an electrostatic precipitator 305, and part of thedust (ash) removed by the electrostatic precipitator 305 is heated byheating means 311 making use of upstream gas of air heater or the liketo sublimate Se in the dust (hexavalent Se and tetravalent Se), therebyadsorbing in an adsorption column 312.

On the other hand, the remaining dust is repulped (dissolved in water)by the liquid from a wet desulfurization apparatus 320 (ORP control:hereinafter desulfurization apparatus 320) by repulping means 313, andformed into slurry, and a treating agent A (such as FeCl₃) from treatingagent feeding means 316 is charged by mixing means 314, and byseparating into solid and liquid in separating means 315, tetravalent Seis solidified and separated into the solid-phase side. On the otherhand, hexavalent Se dissolved in the liquid-phase side in the separatingmeans 315 is led into the desulfurization apparatus 320, and nearly thewhole volume is reduced by the so-called ORP control(oxidation-reduction potential control) to be transformed intotetravalent Se, thereby facilitating the treatment by makingwastewater-free in the wastewater treating apparatus 350.

The heating means 311 is designed to heat the ash introduced from theelectrostatic precipitator 305 up to a temperature for sublimating andgasifying hexavalent Se and tetravalent Se (100 to 1200° C., preferably320 to 1000° C.), and part of produced gas is led into the absorptioncolumn 312, and the rest is sent into the combustion exhaust gas feed-inside of the desulfurization apparatus 320. The ash left over in theheating means 311 is deprived of Se components, and is recycled ascement material. The adsorption column 312 is designed to absorb andcapture part of Se in the produced gas from the heating means 311 so asto be solidified, and herein the gas containing Se which is not adsorbedis sent into the combustion exhaust gas feed-in side of thedesulfurization apparatus 320.

To the repulping means 313, as shown in. FIG. 10, the liquid in afiltrate tank 333 is supplied by means of a pump 334 of thedesulfurization apparatus 320, and the dust introduced from theelectrostatic precipitator 305 is made into slurry by this liquid. Themixing means 314 is, for example, composed of a mixing tank and anagitating mechanism for agitating the liquid in the mixing tank, and.the dust slurry formed by the repulping means 313, the treating agent Afrom the treating agent feeding means 316, and, if necessary, a filteradditive B are charged, and they are mixed and sent into the separatingmeans 315 at the downstream side. As the treating agent A, a chemicalreacting at least with tetravalent Se (main form: selenious acid SeO₃²⁻) to make insoluble is needed, and, for example, FeCl₃ or Fe₂ (SO₄)₃may be used. As the filter additive B, a chemical large in particle sizeand having a function for supporting solid-liquid separation is desired,and, for example, gypsum (gypsum C produced in the desulfurizationapparatus 320) may be used. The separating means 315 is, for example, acentrifugal setting machine, and, in this case, only the filtrate isreturned to the absorbent slurry column 335 of the desulfurizationapparatus 320.

The desulfurization apparatus 320 is of tank oxidation type, andcomprises an absorption column 321 for feeding an absorbent slurry(composed of limestone in this example) into a bottom tank 322, acirculation pump 323 for sending the absorbent slurry in the tank 322into an upper part 321 a (combustion exhaust gas feed-in unit) of theabsorption tank 321 to contact with the combustion exhaust gas, a rotaryarm type air sparger 324 supported in the tank 322 for rotationhorizontally by means of a motor not shown, and agitating the slurry inthe tank 322 and blowing in the supplied aid efficiently into the tank322 as fine bubbles, and an air feed tube 325 for feeding air into thisair sparger 324, and it is designed to obtain gypsum by totallyoxidizing by efficient contact between air and the absorbent slurryabsorbing sulfurous acid in the tank 322.

A slurry pump 331 for sucking out the slurry in the tank 322 isconnected to the tank 322, and the slurry sucked out by this slurry pump331 is concentrated through a thickener not shown, and is supplied intothe solid-liquid separator 332 to be filtered, and the gypsum C in theslurry is taken out as solid cake (usually water content of about 10%).On the other hand, the separated water by the thickener and the filtrate(mainly water) from the solid-liquid separator 332 are once sent intothe filtrate tank 333, and, as required, makeup water D or return liquidE from the wastewater treating apparatus 350 is added, and part of suchliquid is sent into the absorbent slurry tank 335 by a pump 334, andmixed with limestone F (CaCO₃) supplied from a limestone silo not shownto be formed into an absorbent slurry, which is supplied again into thetank 322 by a slurry pump 336.

The desulfurization apparatus 320 is further provided with, as apreferred embodiment of the third invention, oxidation-reductionreaction control means 340 for controlling the oxidation-reductionreaction in the absorption column 321. This oxidation-reduction reactioncontrol means 340 is composed of a sensor 341 disposed in the dischargeside piping of the circulation pump 323 for detecting theoxidation-reduction potential of the slurry in the tank 322, a flow ratecontrol valve 342 disposed in the midst of the air feed tube 325 foradjusting the air feed rate into the air sparger 324, and a controller343 for controlling the action of the flow rate control valve 342 on thebasis of the detection output of the sensor 341. Herein, the sensor 341is composed by immersing an electrode, for example, made of platinuminto slurry. The controller 343 is designed to control the openingdegree of the flow rate control valve 342 continuously, so that the airfeed rate into the air sparger 324 maybe a minimum required limit foroxidizing and digesting the sulfurous acid dissolved in the slurry fromthe combustion exhaust gas. For example, more specifically, on the basisof the correlation of the concentration of sulfurous acid andoxidation-reduction potential, the oxidation-reduction potential whenthe concentration of sulfurous acid is nearly zero is predetermined asthe reference potential, and, by proportional control, when theoxidation-reduction potential detected by the sensor 341 becomes lowerthan this reference potential, the air feed rate is increased accordingto the deviation, and when the oxidation-reduction potential detected bythe sensor 341 becomes higher than this reference potential, the airfeed rate is decreased according to the deviation.

Incidentally, since the oxidation-reduction reaction control means 340is designed to feed a minimum required limit for oxidizing the totalvolume of sulfurous acid, it eventually has a function of inducing anewly total reduction reaction of the other acids contained in theslurry by the sulfurous acid.

That is, in this case, as mentioned later, the slurry supplied into thetank 322 from the absorbent slurry tank 335 contains hexavalent Se (mainform: selenic acid SeO₄ ²⁻), but by the control of the controller 343,it reacts with the sulfurous acid absorbed from the combustion exhaustgas to undergo reduction reaction to be transformed into tetravalent Se(main form: selenious acid SeO₃ ²⁻), which takes place in the absorptioncolumn 321. This reaction is expressed in the following reaction formula(5).

SeO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SO₄ ²⁻  (5)

In this embodiment, the wastewater treating apparatus 350 is a so-calledwastewater-free treating apparatus in a known constitution comprising apretreatment facility 351, an electric dialysis facility 352, asecondary concentrating facility 353, and a solidifying facility 354. Inthis wastewater treating apparatus 350, part of the liquid in thefiltrate tank 333 is supplied by the pump 334 of the desulfurizationapparatus 320, and impurities in this liquid (for example, Se and Cl)are removed mainly by the function of the electric dialysis facility352, and the residue after removal is returned to the filtrate tank 333or absorbent slurry tank 335 of the desulfurization apparatus 320. Theremoved impurities are finally solidified in the solidifying facility354, but at least prior to the solidifying process (for example, at aprior stage of the secondary concentrating facility 353), the treatingagent A for reacting with tetravalent Se (main form: selenious acid SeO₃²⁻) to make it insoluble is mixed into the removed impurities from thetreating agent feeding means 316.

In thus constituted combustion exhaust gas treatment system, first, thesulfurous acid in the combustion exhaust gas after removing the dust,and gasified Se are removed, and gypsum is produced and collected in thefollowing procedure.

That is, the combustion exhaust gas introduced into the absorptioncolumn 321 (including the gas sent from the heating means 311) contactswith the absorbent slurry sprayed form a header pipe 326 by thecirculation pump 323, and the sulfurous acid and gasified Se areabsorbed and removed, and discharged as treated combustion exhaust gasfrom a combustion exhaust gas lead-out unit 321 b.

The sulfurous acid absorbed in the absorbent slurry sprayed from theheader pipe 326 and flowing down through a filler 327 is agitated by theair sparger 324 in the tank 322, and contacts with multiple bubbles tobe oxidized, and further undergoes neutralization reaction to becomegypsum. In the absorption column 321, by the reaction in the reactionformula (5), nearly whole volume of hexavalent Se (main form: selenicacid SeO₄ ²⁻) is transformed into tetravalent Se (main form: seleniousacid SeO₃ ²⁻). Principal reactions taking place in this process (otherthan reaction formula (5) are expressed in reaction formulas (6) to (8).

Absorption Column Combustion Exhaust Gas Lead-in Part

SO₂+H₂O→H⁺+HSO³⁻  (6)

Tank

H⁺+HSO₃ ⁻+½O₂→2H⁺+SO₄ ²⁻  (7)

2H⁺+SO₄ ²⁻+CaCO₃ +H₂O→CaSO₄.2H₂O+CO₂  (8)

Thus, in the tank 322, gypsum (CaSO₄2H₂O), a slight amount of limestone(CaCO₃), and tetravalent Se (main form: selenious acid SeO₃ ²⁻) aresuspended or dissolved, and they are sucked out by the slurry pump 331,and concentrated through a thickener not shown, and supplied into thesolid-liquid separator 332 to be filtered, and gypsum C is obtained in acake form of low water content (usually water content about 10%). Atthis time, meanwhile, tetravalent Se (main form: selenious acid SeO₃²⁻), if not dissolved partly, may be slightly separated and mixed intothe gypsum C, but is mostly sent into the filtrate tank 333 togetherwith the separated water or filtrate.

Consequently, the dust removal treatment in the combustion exhaust gasand the treating action of Se contained much in the dust in thiscombustion exhaust gas treatment system are explained below.

Part of dust (ash) removed by the electrostatic precipitator 305 isheated by the heating means 311 to be gasified, and is partly adsorbedand solidified as mentioned above, while the rest is sent and treated inthe absorption column 321 of the desulfurization apparatus 320. On theother hand, the remainder of the dust removed by the electrostaticprecipitator 305 is repulped (dissolved in water) in the liquid sentfrom the filtrate tank 333 of the desulfurization apparatus 320 by therepulping means 313 to be formed into slurry, and treating agent A and,if necessary, filter additive B are added and mixed by the mixing means314.

At this time, in the mixing means 314, the tetravalent Se (main form:selenious acid SeO₃ ²⁻) contained in the slurry formed by the repulpingmeans 313 (dust slurry) reacts with the treating agent A (e.g. FeCl₃ orFe₂ (SO₄)₃) in the following reaction formulas (1), (2), or (3), (4),and becomes insoluble in a form of iron selenite (Fe₂ (SeO₃)₃).

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ³⁻→Fe₂(SeO₃)₃↓(4)

Accordingly, when the dust slurry is separated into solid and liquid bythe separating means 315, the tetravalent Se is separated in thesolid-phase side as iron selenite, and mixed in insoluble form into thedust cake G discharged from the separating means 315. On the other hand,the hexavalent Se in the dust slurry is dissolved in the liquid-phaseside and is contained in the filtrate, and is mixed, in this case, intothe absorbent slurry tank 335 of the desulfurization apparatus 320. Inthe separating means 315, by the function of the filter additive B(gypsum, etc.) charged in the mixing means 314, effective dehydration isrealized, and a low water content in the dust cake G is achieved. At thesame time, the hexavalent Se contained in the filtrate in the separatingmeans 315 and mixed in the adsorbent slurry tank 335 of thedesulfurization apparatus 320 is also contained in the absorbent slurryand sent into the absorption column 321 of the desulfurization apparatus320 with the pump 336, and therefore almost all volume thereof istransformed into tetravalent Se in the reaction (reaction formula (5).

The function of the wastewater treating apparatus 350 in this combustionexhaust gas treatment system is described below. As mentioned above,mainly hexavalent Se is sequentially mixed into the slurry solutioncirculating in the desulfurization apparatus 320 as being contained inthe separated water of the dust slurry, and when this hexavalent Seenters the absorption column 321, it is almost completely transformedinto tetravalent Se by the above reaction (reaction formula 5), and instationary state, therefore, much tetravalent Se mainly formed byreduction of hexavalent Se is present in the slurry solution circulatingin the desulfurization apparatus 320. In this case, the wastewatertreating apparatus 350 functions to remove this tetravalent Se, likeother impurities (e.g. Cl), so as not to be accumulated excessively inthe slurry solution circulating in the desulfurization apparatus 320.

That is, in the wastewater treating apparatus 350, part of the slurrysolution circulating in the desulfurization apparatus 320 is extractedfrom the discharge side of the pump 334, and the impurities in thissolution (Se, Cl, etc.) are removed mainly by the function of theelectric dialysis facility 352, and returned to the filtrate tank 333 ofthe desulfurization apparatus 320. The removed impurities are mixed withthe treating agent A charged from the treating agent feeding means 316,and concentrated in the secondary concentration apparatus 353,solidified by the solidifying facility 354, and discarded in the ashdisposal yard or the like as impurity chip H. At this time, thetetravalent Se in the impurities reacts with the treating agent A in theformulas (1), (2), or (3), (4), and is transformed into iron selenite(Fe₂ (SeO₃)₃), and is present in an insoluble form in the impurity chipH.

As described herein, according to the combustion exhaust gas treatmentsystem of the embodiment, together with the conventional purification ofcombustion exhaust gas (removal of dust, removal of sulfurous acid), Sein the combustion exhaust gas is removed along with dust, and finally itis contained, in an insoluble form, in the dust cake G or impurity chipH, so as to be discarded. Moreover, hexavalent Se which is hard to betreated (made insoluble) is transformed into tetravalent Se which iseasy to discard, by the treating agent by the oxidation-reductionreaction control means 340 in the absorption column 321 of thedesulfurization apparatus 320, and therefore, as compared with thesystem comprising an independent reaction column for transforminghexavalent Se into tetravalent Se, for example, Se in combustion exhaustgas may be removed and made harmless easily and inexpensively.

Still more, as the solvent for repulsing the dust captured by theelectrostatic precipitator 305 by the repulsing means 313, by using partof the circulation liquid of the desulfurization apparatus 320, thewater flow (circulation volume) and consumption are saved as comparedwith the system of feeding water separately into the repulping means313, and the running cost of the system is curtailed, and at the sametime the required capacity of the wastewater treating apparatus 350 isdecreased, so that the facility cost may be further reduced.

When the mixing means 314 is designed to charge filter additive B, thedehydration rate is heightened in the separating means 315, and dustcake G of lower water content is formed, so that carrying or handling ofdust cake G may be easier.

Moreover, according to this combustion exhaust gas treatment system, bythe function of the oxidation-reduction reaction control means 340,nearly all of hexavalent Se is eventually transformed into tetravalentSe, and is made insoluble and discarded, and hence the concentration ofhexavalent Se remaining in the dust cake G or impurity chip H (not madeinsoluble) is very slight, and the elution standard is satisfied with asufficient margin.

Embodiment 2

FIG. 11 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (4) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 11, the dust captured by the electrostatic precipitator 305 isdirectly introduced into the absorption column 321 of thedesulfurization apparatus 320, and the treating agent A is charged intothe slurry extracted from the absorption column 321 of thedesulfurization apparatus 320 by the treating agent feeding means 316 inthe mixing means 314.

In this case, all Se in the dust once enters the absorption column 321except for the portion extracted by the heating means 311, and in theabsorption column 321, too, the hexavalent Se is transformed intotetravalent Se, and this tetravalent Se is made insoluble by thetreating agent A, and is mixed into the gypsum C or the impurity chip Hin the wastewater treating apparatus 350. Herein, the absorption column321 also functions as the repulping means 313 in embodiment 1 and thesolid-liquid separator 332 also functions as the separating means 315 inembodiment 1, and as compared with the system in embodiment 1,therefore, the repulping means 313 and separating means 315 are notnecessary, thereby contributing further to reduction of the facilitycost.

In this embodiment, meanwhile, although the cost may be further saved ascompared with embodiment 1, but due to the effect of massive dust(impurities) mixing into the absorption column, it may be difficult tokeep a high desulfurization rate or high quality of gypsum C, and wherethis problem is feared, embodiment 1 or embodiment 4 or 5 mentionedlater may be preferable, and in this respect the constitution ofembodiment 1 is superior. Incidentally, the treating agent A may bemixed into other position than the position shown in FIG. 11, as far aswithin the slurry system of the desulfurization apparatus 320, or may bedirectly mixed into the absorption column 321.

Embodiment 3

FIG. 12 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (5) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 12, without installing electrostatic precipitator 305, combustionexhaust gas is directly fed into the absorption column 321 of thedesulfurization apparatus 320, together with fly ash and dust, and thetreating agent A is charged into the slurry extracted from theabsorption column 321 of the desulfurization apparatus 320 from thetreating agent feeding means 316 in the mixing means 314.

In this case, all Se in the dust once enters the absorption column 321,and in the absorption column 321, the hexavalent Se is transformed intotetravalent Se, and this tetravalent Se is made insoluble by thetreating agent A, and is mixed into the gypsum C or the impurity chip Hin the wastewater treating apparatus 350. Herein, the absorption column321 also functions as the electrostatic precipitator 305 and repulpingmeans 313 in embodiment 1, and the solid-liquid separator 332 alsofunctions as the separating means 315 in embodiment 1, and as comparedwith the system in embodiment 1, therefore, the electrostaticprecipitator 305, repulping means 313 and separating means 315 are notnecessary, thereby contributing further to reduction of the facilitycost.

In this embodiment, meanwhile, although the cost may be further saved ascompared with embodiment 1, but due to the effect of massive dust(impurities) mixing into the absorption column, it may be difficult tokeep a high desulfurization rate or high quality of gypsum C, and wherethis problem is feared, embodiment 1 or embodiment 4 or 5 mentionedlater may be preferable, and in this respect the constitution ofembodiment 1 is superior.

Yet, since the electrostatic precipitator 305 is not provided, thebyproduct obtained in the heating means 311 is slight, and where thisproblem is feared, the constitution of embodiment 1, 2 or 4 may bepreferred, and in this respect the constitution of embodiment 1 or othermay be superior. In this constitution, too, the mixing position of thetreating agent A is not limited to the position shown in FIG. 12, but itmay be mixed in any arbitrary position in the slurry system of thedesulfurization apparatus 320, or may be directly charged into theabsorption column 321.

Embodiment 4

FIG. 13 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (7) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 13, a desulfurization apparatus 360 having a cooling and dustcollecting column 361 for cooling the dust and removing dust is disposedat the upstream side of the absorption column 321, and the dust capturedby the electrostatic precipitator 305 is directly fed into the coolingand dust collecting column 361 of the desulfurization apparatus 360,while the treating agent A is charged into the slurry extracted from thecooling and dust removing column 361 from the treating agent feedingmeans 316 in the mixing means 314. Herein, in the cooling and dustcollecting column 361, the liquid from the filtrate tank 333 is suppliedfrom the pump 334, and this liquid is sprayed from an upper header pipe363 by a circulation pump 362. Between the cooling and dust collectingcolumn 361 and absorption column 321, a mist eliminator, not show, isprovided.

In this case, all Se in dust once gets into the cooling and dustremoving column 361, and the reaction in (reaction formula (5)) occursin the cooling and dust collecting column 361, and hexavalent Se isalmost totally transformed into tetravalent Se, and this tetravalent Seis made insoluble by the treating agent A, and is mixed into the dustcake G or impurity chip H. Herein, the cooling and dust collectingcolumn 361 also functions as the repulping means 313 in embodiment 1,and therefore the repulping means 313 is not needed as compared withembodiment 1, and the facility cost may be reduced.

In this embodiment, moreover, different from embodiments 1 to 3, massivedust does not mix into the absorption column 321, and therefore thefacility cost may be further reduced, and the high desulfurization ratemay be kept and gypsum C of high quality may be realized.

In FIG. 13, meanwhile, the separated water of the separating means 315is directly fed into the wastewater treating apparatus 350, but forfurther perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 335 and then guided into the absorption column321.

The treating agent A may be also charged directly into the cooling anddust collecting column 361.

Embodiment 5

FIG. 14 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (8) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 14, a desulfurization apparatus 360 having a cooling and dustcollecting column 361 for cooling the dust and removing dust is disposedat the upstream side of the absorption column 321, and the combustionexhaust gas not being rid of dust is directly fed into the cooling anddust collecting column 361 of the desulfurization apparatus 360, whilethe treating agent A is charged into the slurry extracted from thecooling and dust removing column 361 from the treating agent feedingmeans 316 in the mixing means 314.

In this case, all Se in dust once gets into the cooling and dustremoving column 361, and the reaction in (reaction formula (5)) occursin the cooling and dust collecting column 361, and hexavalent Se isalmost totally transformed into tetravalent Se, and this tetravalent Seis made insoluble by the treating agent A, and is mixed into the dustcake G or impurity chip H. Herein, the cooling and dust collectingcolumn 361 also functions as the electrostatic precipitator 305 andrepulping means 313 in embodiment 1, and therefore the electrostaticprecipitator 305 and repulsing means 313 are not needed as compared withembodiment 1, and the facility cost may be reduced.

In this embodiment, moreover, different from embodiments 2 and 3,massive dust does not mix into the absorption column 321, and thereforethe facility cost may be further reduced, and the high desulfurizationrate may be kept and gypsum C of high quality may be realized.

In FIG. 14, meanwhile, the separated water of the separating means 315is directly fed into the wastewater treating apparatus 350, but forfurther perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 335 and then guided into the absorption column321.

Yet, since the electrostatic precipitator 305 is not provided, thebyproduct obtained in the heating means 311 is slight, and where thisproblem is feared, the constitution of embodiment 1, 2 or 4 may bepreferred, and in this respect the constitution of embodiment 1 or othermay be superior.

The third invention may be also realized in many other forms aside fromthe foregoing embodiments. For example, if hexavalent Se does not existand only tetravalent Se is present in the combustion exhaust gas, theprocess or apparatus for reducing hexavalent Se into tetravalent Se isnot needed. The process and apparatus for heating and recycling part ofthe dust removed by the dust collector by heating means may be providedonly where necessary.

The constitution of the desulfurization apparatus is not limited to thetank oxidation type shown in the embodiments, and, for example, anoxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

The repulping means and mixing means in embodiment 1 are not alwaysrequired to be composed of independent tank, but, for example, it may bealso designed to repulp the dust and mix the treating agent in one tank(that is, the repulping means and mixing means of the invention may becomposed of a single tank).

In embodiments 2 to 5, the charging position of the treating agent A maybe a position in the wastewater treating apparatus 350. That is, sincethe slurry liquid in the absorption column or cooling and dustcollecting column of the desulfurization apparatus is circulating in thewastewater treating apparatus 350, the entire Se can be made insolubleby charging the treating agent only in the wastewater treating apparatus350.

Effects of the Third Invention

According to the combustion exhaust gas treatment system of the thirdinvention (1), if hexavalent Se is contained in the combustion exhaustgas, all Se (both hexavalent and tetravalent) removed from thecombustion exhaust gas as dust can be finally discharged as tetravalentSe, and the Se elution standard may be satisfied easily only by treatingwith the treating agent to be insoluble, and the absorption column ofthe desulfurization apparatus also functions as the reduction reactionfacility of hexavalent Se, and the facility constitution of the entiresystem is facilitated as compared with the constitution of installing anindependent reaction column for reducing Se.

According to the combustion exhaust gas treatment system of the thirdinvention (2), since part of the circulation liquid of thedesulfurization apparatus can be used as solvent in the repulping means,the water flow (circulation) and consumption can be saved as comparedwith the constitution for feeding water separately.

According to the combustion exhaust gas treatment system of the thirdinvention (3), since filter additive is charged into the mixing means orseparating means, the dehydration performance in the separating means isenhanced, the solid matter (dust cake) of low water content and easy tohandle is obtained.

According to the combustion exhaust gas treatment system of the thirdinvention (4), the Se elution standard may be easily satisfied, and thedesulfurization apparatus functions also as hexavalent Se reductionreaction facility or as dust repulping means, so that the constitutionof the entire system is simplified as compared with the systemcomprising Se reduction reaction column or repulping means separately.

According to the combustion exhaust gas treatment system of the thirdinvention (5), the Se elution standard may be easily satisfied, and thedesulfurization apparatus. functions also as dust collector, orhexavalent Se reduction reaction facility, or dust repulping means, sothat the constitution of the entire system is simplified as comparedwith the system comprising dust collector, Se reduction reaction columnor repulping means separately.

According to the combustion exhaust gas treatment system of the thirdinvention (6), the oxidation-reduction reaction control means controlsthe oxidation-reduction reaction of the slurry in the desulfurizationapparatus, so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost completely reduced intotetravalent form by the sulfurous acid in the slurry. Accordingly, ifhexavalent Se is contained in the combustion exhaust gas, thishexavalent Se can be almost completely transformed into tetravalent formin the desulfurization apparatus, so that the Se in the combustionexhaust gas may be made insoluble more easily and perfectly.

According to the combustion exhaust gas treatment system of the thirdinvention (7), the Se elution standard may be easily satisfied, andmoreover since the desulfurization apparatus functions also ashexavalent Se reducing reaction facility, the constitution of the entiresystem is simplified as compared with the system comprising reactioncolumn for reducing Se separately. Also in this case, since the coolingand dust removing column also function as dust repulping means, theconstitution of the entire system is more simplified as compared withthe system comprising repulping means separately. More preferably, sincemuch dust (Se and other impurities) does not mix into the slurry in theabsorption column of the desulfurization apparatus, the performance suchas desulfurization rate in the desulfurization apparatus can bemaintained high.

According to the combustion exhaust gas treatment system of the thirdinvention (8), the Se elution standard may be easily satisfied, and thedesulfurization apparatus functions also as dust collector, orhexavalent Se reduction reaction facility, or dust repulping means, sothat the constitution of the entire system is simplified as comparedwith the system comprising dust collector, Se reduction reaction columnor repulping means separately. More preferably, since much dust (Se andother impurities) does not mix into the slurry in the absorption columnof the desulfurization apparatus, the performance such asdesulfurization rate in the desulfurization apparatus can be maintainedhigh.

According to the combustion exhaust gas treatment system of the thirdinvention (9), the Se eluting into wastewater in the desulfurizationapparatus can be also made insoluble, and more perfect Se insolubletreatment is realized, and Se-free gypsum can be collected. Depending onthe treating conditions, moreover, the insoluble treating agent may becharged only into the wastewater treating apparatus, so that the entiresystem can be simplified.

D: Fourth Invention

Referring now to the drawings, embodiments of the fourth invention aredescribed below.

Embodiment 1

FIG. 15 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system according to the fourthinvention (1) to (3),and (5) to (7).

In the combustion exhaust gas treatment system of this embodiment, asshown in FIG. 15, the dust containing Se is removed by an electrostaticprecipitator 470, the dust (ash) removed by the electrostaticprecipitator 470 is heated by heating means 411 to sublimate and gasifythe Se in the dust, and this gas is fed into a wet desulfurizationapparatus 420 (hereinafter, desulfurization apparatus 420), togetherwith the combustion exhaust gas being rid of dust, to be treated.

In the desulfurization apparatus, as shown in reaction formulas (9) and(10), tetravalent Se (main form: selenious acid SeO₃ ²⁻) and hexavalentSe (main form: selenic acid SeO₄ ²⁻) are present. This hexavalent Se isalmost completely reduced in the desulfurization apparatus 420 byso-called ORP control (oxidation-reduction potential control) to betransformed into tetravalent Se.

SeO_(2(g))+H₂O→2H⁺+SeO₃ ²⁻  (9)

SeO₃ ²⁻+½O₂→SeO₄ ²⁻  (10)

Herein, the heating means 411 is, for example, a kiln, which is designedto heat the ash B3, B4 introduced from the electrostatic precipitator470 until Se is sublimated and gasified by the supplied hot air I, andproduced gas M is fed into the absorption column 421 of thedesulfurization apparatus together with the combustion exhaust gas,while the remaining ash J is taken out to be recycled as cement materialor the like. As means for feeding hot air I into the heating means 411,in this case, a heavy oil fired boiler 412 is provided, and by burningheavy oil K and heating the air L, hot air I at about, for example,1000° C. is supplied.

The treating temperature in the heating means 411 may be 100 to 1200°C., but in order to gasify the Se in the dust more perfectly andefficiently, it is preferred to set in a range of 320 to 1000° C. Forinstance, when the flow and temperature of the dust introduced into theheating means 411 from the electrostatic precipitator 470 are 15 t/h and90° C., to heat to about 320° C., the flow and temperature of hot air I,and the kiln specification for composing the heating means 411 may beset, for example, as follows. That is, the flow rate of hot air I is3100 m³ N/h, temperature of hot air I is 1000° C., the filling rate inthe kiln is 8%, the ash density in kiln is 0.5, and the residence timein the kiln is 0.5 hr.

The electrostatic precipitator 470 comprises plural hoppers 471 to 474for separating and collecting the dust, and these hoppers 471 to 474 areformed sequentially from the inlet side (upstream side) to the outletside (downstream side) of the combustion exhaust gas, and in thisconstitution, dust of larger particle size is collected from the inletside hopper, and dust of smaller particle size is collected from theoutlet side hopper. In this case, only the dust B3, B4 separated andcollected. from the specific hoppers 473, 474 at the outlet side are fedinto the heating means 411, and the remaining dust B1, B2 are recycledas cement material or discarded directly. In this embodiment, meanwhile,the electrostatic precipitator 470 functions as the dust collectingmeans and. sorting means of the fourth invention.

The desulfurization apparatus 420 is of tank oxidation type in thisembodiment, and comprises an absorption tank 421 in which an absorbentslurry (composed of limestone in this case) is supplied into a bottomtank 422, a circulation pump 423 for sending the absorbent slurry in thetank 422 into an upper part 421 a (combustion exhaust gas lead-in part)of the absorption tank 421 to contact with combustion exhaust gas, anarm-rotary type air sparger 424 supported in the tank 422 for rotatinghorizontally by a motor not shown, agitating the slurry in the tank 422,and blowing the supplied air efficiently into the tank 422 as finebubbles, and an air feed pipe 425 for feeding air into the air sparger424, in which the absorbent slurry absorbing sulfurous acid and the airare brought into contact efficiently in the tank 422, and the wholevolume is oxidized to obtain gypsum.

In this tank 422, a slurry pump 431 for sucking out the slurry in thetank 422 is connected, and the treating agent A is charged by the mixingmeans 414 into the slurry sucked out by this slurry pump 431, and themixture is supplied into a solid-liquid separator 432 to be filtered,and the gypsum C in the slurry is taken out as solid cake (usually watercontent about 10%). On the other hand, the filtrate (mainly water) fromthe solid-liquid separator 432 is once sent into the filtrate tank 433,and, if necessary, makeup water D or return liquid E from the wastewatertreating apparatus 450 is added, and part of the liquid is sent into theadsorbent slurry tank 435 by the pump 434, and mixed with limestone F(CaCO₃) supplied from limestone silo not shown, and is supplied asabsorbent slurry again into the tank 422 by the slurry pump 436. Themixing means 414 is composed of, for example, a mixing tank and anagitating mechanism for agitating the liquid in the mixing tank. As thetreating agent A, a chemical at least reacting with tetravalent Se (mainform: selenious acid SeO₃ ²⁻) to make it insoluble is needed, and forexample, FeCl₃ or Fe₂ (SO₄)₃ may be used.

The desulfurization apparatus 420 is further equipped withoxidation-reduction reaction control means 440 for controlling theoxidation-reduction reaction in the absorption column 421. Theoxidation-reduction reaction control means 440 consists, in this case,of a sensor 441 provided in the discharge side piping of the circulationpump 423 for detecting the oxidation-reduction potential of the slurryin the tank 422, a flow control valve 442 provided on the way of airfeed pipe 425 for adjusting the air supply flow into the air sparger424, and a controller 443 for controlling the action of the flow controlvalve 442 on the basis of the detection output of the sensor 441.Herein, the sensor 441 is realized by immersing an electrode made of,for example, platinum in the slurry. The controller 443 is designed tocontrol continuously the opening degree of the flow control valve 442,so that the air feed rate into the air sparger 424 may be a minimumrequired limit for oxidizing and digesting the sulfurous acid dissolvedin the slurry from the combustion exhaust gas. More specifically, on thebasis of the correlation of the sulfurous acid concentration andoxidation-reduction potential, the oxidation-reduction potential whenthe sulfurous acid concentration is nearly zero is predetermined as thereference potential, and the proportional control is effected toincrease the air feed rate depending on the deviation when theoxidation-reduction potential detected by the sensor 441 is lower thanthis reference potential, and to decrease the air feed rate depending onthe deviation when the oxidation-reduction potential detected by thesensor 441 is higher than the reference potential.

Incidentally, since the oxidation-reduction reaction control means 440is designed to supply air of the minimum required limit for oxidizingthe total volume of sulfurous acid, it eventually has a function ofinducing the reaction for reducing almost whole volume of other acidscontained in the slurry by the sulfurous acid.

That is, the gas M containing vaporized Se released from the heatingmeans 411 is fed into the absorption column 421 together with combustionexhaust gas, and becomes tetravalent Se (main form: selenious acid SeO₃²⁻) and hexavalent Se (main form: selenic acid SeO₄ ²⁻), but by thecontrol of the controller 443, the hexavalent Se reacts with thesulfurous acid absorbed from the combustion exhaust gas to becometetravalent Se (main form: selenious acid SeO₃ ²⁻) in the reductionreaction, which takes place in the absorption column 421. This reactionis expressed in the following reaction formula (11).

SeO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SO₄ ²⁻  (11)

The wastewater treating apparatus 450 is a so-called wastewater-freetreating apparatus, comprising a pretreatment facility 451, an electricdialysis facility 452, a secondary concentrating facility 453, and asolidifying facility 454. In this wastewater treating apparatus 450,part of the liquid in the filtrate tank 433 is supplied by the pump 434of the desulfurization apparatus 420, and impurities in this liquid (forexample, Cl) are removed mainly by the function of the electric dialysisfacility 452, and the residue after removal is returned to the filtratetank 433 or absorbent slurry tank 435 of the desulfurization apparatus420. The removed impurities are finally solidified in the solidifyingfacility 454, but at least prior to the solidifying process (forexample, at a prior stage of the secondary concentrating facility 453),the treating agent A for reacting with tetravalent Se (main form:selenious acid SeO₃ ²⁻) to make it insoluble is mixed in.

In thus constituted combustion exhaust gas treatment system, thecombustion exhaust gas is sufficiently cooled upstream of theelectrostatic precipitator 470, and Se in the combustion exhaust gas ismostly condensed and deposits on the fly ash or other dust (particularlyon small particles), and therefore most Se in combustion exhaust gas iscaptured once by the electrostatic precipitator 470 together with thedust. In this case, of the captured dust B1 to B4, dust B1 and B2 oflarge particle size are small in Se content, and may be hence directlyused as cement material or discarded, while only dust B3 and B4 of smallparticle size are heated in the heating means 411, and the Se in thedust B3, B4 is gasified, and fed into the absorption column 421 of thedesulfurization apparatus 420 together with the combustion exhaust gasreleased from the electrostatic precipitator 470.

The combustion exhaust gas introduced into the absorption column 421(including the gas sent from the heating means 411 and others) contactswith the absorbent slurry sprayed from a spray valve 426 by thecirculation pump 423, and the sulfurous acid and gasified Se areabsorbed and removed, and is discharged from the combustion exhaust gaslead-out part 421 b as treated combustion exhaust gas.

The sulfurous acid sprayed from the spray valve 426 and absorbed in theabsorbent slurry flowing down through a filler 427 is agitated by theair sparger 424 in the tank 422 and contacts with multiple bubbles blownin to be oxidized, and further undergoes neutralization reaction tobecome gypsum. In the absorption column 421, by the reaction of thereaction formula (11), nearly whole volume of hexavalent Se (main form:selenic acid SeO₄ ²⁻) is transformed into tetravalent Se (main form:selenious acid SeO₃ ²⁻). Principal reactions (except for reactionformula 11) taking place in this process are expressed in the followingreaction formulas (12) to (14).

Absorption Column Combustion Exhaust Gas Lead-in Part

SO₂+H₂O→H⁺HSO₃ ⁻  (12)

Tank

H⁺+HSO₃ ⁻+½O₂→2H⁺+SO₄ ²⁻  (13)

2H⁺+SO₄ ²⁻+CaCO₃+H₂O→CaSO₄.2H₂O+CO₂  (14)

Thus, in the tank 422, gypsum (CaSO₄.2H₂O), a small amount of limestone(CaCO₃) as absorbent, and mainly tetravalent Se (main form: seleniousacid SeO₃ ²⁻) are suspended, and they are sucked out by the slurry pump431, and the treating agent A is mixed by the mixing means 414, and themixture is supplied into the solid-liquid separator 432 to be filtered,and gypsum C is obtained as cake form of low water content (usuallywater content about 10%).

Most of tetravalent Se (main form: selenious acid SeO₃ ²⁻) undergoes thereaction expressed in reaction formulas 1, 2, or 3, 4, and is madeinsoluble in a form of iron selenite (Fe₂(SeO₃)₃), and is mixed in theseparated gypsum C.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ³⁻→Fe₂(SeO₃)₃↓(4)

If, however, it is not desired that iron selenite (Fe₂ (SeO₃)₃) is mixedin the separated and collected gypsum C, the slurry from the slurry pump431 is supplied directly. into the solid-liquid separator 432 through aline 437 (shown in FIG. 15), and gypsum C of high purity is recovered.In this case, Se is treated to be insoluble in the wastewater treatingapparatus 450 shown below.

The function of the wastewater treating apparatus 450 in the combustionexhaust gas treatment system is described below.

As mentioned above, the vaporized Se is absorbed in the slurry in thedesulfurization apparatus 420 together with the sulfurous acid in thecombustion exhaust gas, and the hexavalent Se thereof reacts (reactionformula (11) in the absorption column 421 and is almost completelytransformed into tetravalent Se. This tetravalent Se is treated same asother impurities (e.g. Cl) in the wastewater treating apparatus 450, andthis Se and other impurities are removed so as not to be accumulatedexcessively in the slurry solution circulating in the desulfurizationapparatus 420.

That is, in the wastewater treating apparatus 450, part of the filtrateof the filtrate tank 433 in the desulfurization apparatus 420 isextracted from the discharge side of the pump 434, and the impurities inthis solution (Cl, etc.) are removed mainly by the function of theelectric dialysis facility 452, and returned to the filtrate tank 433 ofthe desulfurization apparatus 420. The liquid leaving the electricdialysis facility 452 is mixed with the treating agent, and the mixtureis concentrated in the secondary concentration apparatus 453, solidifiedby the solidifying facility 454, and discarded in the ash disposal yardor the like as impurity chip H. At this time, the tetravalent Se in theimpurities reacts with the treating agent A in the formulas (1), (2), or(3), (4), and is transformed into iron selenite (Fe₂ (SeO₃)₃ ), and ispresent in an insoluble form in the impurity chip H.

As described herein, according to the combustion exhaust gas treatmentsystem of the embodiment, in addition to the conventional purificationof combustion exhaust gas (removal of dust, removal of sulfurous acid),Se in the combustion exhaust gas is removed along with dust, and finallyit is contained, in an insoluble form, in the dust cake G or impuritychip H, so as not to be eluted when recycled or discarded. Moreover,hexavalent Se which is hard to be treated (made insoluble) istransformed into tetravalent Se which is easy to discard, by thetreating agent by the oxidation-reduction reaction control means 440 inthe absorption column 421 of the desulfurization apparatus 420, andtherefore, as compared with the system comprising an independentreaction column for transforming. hexavalent Se into tetravalent Se, forexample, Se in combustion exhaust gas may be removed and made harmlesseasily and inexpensively.

What is more, according to the combustion exhaust gas treatment system,by the function of the oxidation-reduction reaction control means 440,nearly whole volume of hexavalent Se is eventually transformed intotetravalent Se in the absorption column 421, and is finally madeinsoluble and discarded, and therefore the concentration of Se (not madeinsoluble) remaining in the gypsum cake C or impurity chip H istrifling, and the elution standard may be satisfied with an amplemargin.

In this case, moreover, by applying Se insoluble treatment only on thedust B3, B4 separated and collected from the specific hoppers 473, 474at the outlet side of the combustion exhaust gas in the electrostaticprecipitator 470, the required amount of the treating agent A and therequired capacity of the heating means 411 may be reduced, so that theSe may be made harmless more easily and inexpensively.

That is, according to the study by the present inventors, it is knownthat more Se is contained (deposited) in the smaller particle size dust(ash) separated and collected from the specific recovery unit at theoutlet side, and the Se is made harmless on the whole only by applyinginsoluble treatment on the dust of smaller particle size, therebycontributing to reduction of facility cost and running cost.

Below are explained the results of dusts heating experiment, dustcapturing test, and elution experiment, in the same apparatus as in theabove embodiment.

The heating experiment was conducted by heating the dust containing 84mg/kg of Se at various temperatures (200 to 1200° C.) for variousdurations (5 to 30 minutes). Before and after heating experiment, the Seelution test of dust was conducted in a method conforming to ordinanceNo. 13 of the Environmental Agency of Japan, and the Se concentration inthe elution solution was analyzed by the atomic absorption photometry bythe hydrogen compound generating method.

The results by heating temperature are shown in FIG. 18, and effects ofh eating time are given in FIG. 19. When heated for 10 to 30 minutes attemperature of 320° C. or more, the Se elution in the dust was less thanthe elution standard of 0.3 mg/liter concerning the landfill regulation,and it is known that Se is mostly gasified. At temperature of 200° C.,by extending the heating time to 30 minutes, the Se elution from thedust was below the elution standard concerning landfill.

Therefore, when gasifying the Se in dust by heating, as the temperaturenot allowing the gasified Se to condense again, by heating the dusttemperature to 100 to 1200° C., preferably 320 to 1000° C., the Se inthe dust can be removed. That is, in this embodiment, the dust J afterbeing heated by the heating means 411 is proved to be recycled ordiscarded directly.

In the dust capturing and elution test, coal containing 3 mg/kg of Sewas supplied into a combustion furnace at a rate of 25 kg/h, and thecombustion exhaust gas exhausted at 200 m³ N/h from the combustionfurnace was cooled to 150° C., and fed into the electrostaticprecipitator. In this case, more than 99% of the dust was captured bythe electrostatic precipitator, and the total volume collected from allhoppers was 3.4 kg/h. Discharge, mean particle size, and eluting Seconcentration in the dust B1, B2 (collected ash) collected from theupstream hoppers 471, 472, and dust B3, B4 collected from downstreamhoppers 473, 474 were measured, of which results are shown in Table 9.

TABLE 9 Collected ash Collected ash Collected ash discharged dischargedfrom hoppers from hoppers 471, 472 473, 474 (combustion exhaust(combustion exhaust gas inlet gas Item side) outlet side) Dischargeamount kg/h 2.27 1.14 Mean particle size of 12 5 collected ash μmEluting selenium 0.20 0.49 concentration in collected ash mg/liter

More specifically, the discharge amount of the dust B3, B4 separated andcollected from the hoppers 473, 474 at the outlet side of the combustionexhaust gas was slight, 1.14 kg/h, but the eluting Se concentration was0.49 mg/liter, high above the standard. On the other hand, the dischargeamount of the dust B1, B2 separated and collected from the hoppers 471,472 at the inlet side of combustion exhaust gas was large, 2.27 kg/h,but the eluting Se concentration was 0.20 mg/liter, far below thestandard. Accordingly, it is known that the dust B1, B2 separated andcollected from the hoppers 471, 472 at the inlet side of the combustionexhaust gas can be directly discarded. That is, Se insoluble treatmentis not needed in the dust at the inlet side of the combustion exhaustgas which is about twice larger in the discharge amount, and hence it isevident that the required amount of the treating agent A and therequired capacity of the heating means 411 can be substantially saved.

Incidentally, such difference in eluting Se concentration is regarded tobe due to the particle size of dust (ash). That is, when gaseous Se(SeO₂) is condensed and deposits on the surface of the ash forming thedust, ash of smaller particle size is greater in the specific surfacearea per unit weight, and hence more Se deposits. On the other hand, inthe dust collector such as the electrostatic precipitator mentionedabove, coarse ash particles are likely to be captured at the inlet sideof the combustion exhaust gas, and fine ash particles are likely to becaptured at the outlet side of the combustion exhaust gas, and in otherwords there is a sorting function, and it is hence considered that theeluting Se concentration is high in the dust captured at the outlet sideof the combustion exhaust gas.

In this embodiment, due to the effect of Se mixing into the absorptioncolumn, it may be difficult to realize a high quality (purity) of gypsumC, and if this is a problem, as mentioned above, Se insoluble treatmentmay be done only in the wastewater treating apparatus 450.

Incidentally, the treating agent A may be mixed in other position thanthe position shown in FIG. 15 as far as within the slurry system of thedesulfurization apparatus 420, or may be directly charged into theabsorption column 421. Or the treating agent A may be mixed only in thewastewater treating apparatus 450 of the desulfurization apparatus 420,and hence the mixing means 414 in FIG. 15 may be omitted. In this case,in the desulfurization apparatus 420, all Se (especially tetravalent Se)circulates as being dissolved in the slurry solution, and part of the Seis sequentially led into the wastewater treating apparatus 450 to bemade insoluble, and hence does not mix into the gypsum C, so that it isbeneficial when desired to keep a high purity of gypsum.

Moreover, if it is not necessary to reduce the charging amount of thetreating agent A or required capacity of the heating means 411, all ofthe dust B1 to B4 collected in the electrostatic precipitator 470 may befed into the heating means 411 and treated.

Embodiment 2

FIG. 16 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the fourth invention (4).Same constituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

In the combustion exhaust gas treatment system of the embodiment, asshown in FIG. 16, a desulfurization apparatus 460 having a cooling anddust collecting column 461 for cooling the dust and removing dust isdisposed at the upstream side of the absorption column 421, and the gasincluding Se generated in heating means 411, together with combustionexhaust gas, is fed into the cooling and dust collecting column 461 ofthe desulfurization apparatus 460, while the treating agent A is chargedinto the slurry extracted from the cooling and dust removing column 461by the mixing means 414. and it is separated into solid and liquid bythe separating means 415.

Herein, in the cooling and dust collecting column 461, the liquid fromthe filtrate tank 433 is supplied from the pump 434, and this liquid issprayed from an upper header pipe 463 by a circulation pump 462. Betweenthe cooling and dust collecting column 461 and absorption column 421, amist eliminator, not show, is provided.

In this case, the Se separated from the dust by heating once gets intothe cooling and dust removing column 461, and the reaction in (reactionformula (11)) occurs in the cooling and dust collecting column 461, andhexavalent Se is almost totally transformed into tetravalent Se, andthis tetravalent Se is made insoluble by the treating agent A, and ismixed into the dust cake G or impurity chip H in the wastewater treatingapparatus 450. Herein, different from embodiment 1, fine dust notcaptured by the electrostatic precipitator does not mix into theabsorption column 421, and therefore a high desulfurization rate may bekept and gypsum C of high quality may be easily obtained.

In FIG. 16, meanwhile, the separated water of the separating means 415is directly fed into the wastewater treating apparatus 450, but forfurther perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 435 and then guided into the absorption column421.

The treating agent A may be also charged directly into the cooling anddust collecting column 461, or same as in embodiment 1, the treatingagent A may be charged only into the wastewater treating apparatus 450,and the mixing means 414 and separating means 415 may be omitted.

Embodiment 3

FIG. 17 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the fourth invention (6).Same constituent elements as in embodiment 1 are identified with samereference numerals and their explanations are omitted.

This combustion exhaust gas treatment system, as shown in FIG. 17,comprises a sorter 481 (sorting means) for classifying the dust B1 to B4captured by the electrostatic precipitator 470 and conveyed in batchinto large particle size (coarse ash) B5 and small particle size (fineash) B6, and only fine ash B6 sorted by the sorter 481 is captured by afine particle capturing apparatus 482, and fed into the heating means411.

Meanwhile, in this case, the dust B1 to B4 are conveyed in batch by airN and led into the sorter 481. The sorter 481 may be constituted by, forexample, a cyclone, and it is convenient when it is designed to adjustthe degree of sorting. As the fine particle capturing apparatus 482, inthis case, a bag filter is used.

In this case,. only the fine ash B6 is gasified and fed into thedesulfurization apparatus 420 to be made, and therefore, same as inembodiment 1, the required amount of treating agent A and requiredcapacity of heating means 411 can be reduced, so that the Se may be madeharmless more easily and inexpensively.

In this combustion exhaust gas treatment system, only by adding theheating means and sorter to the conventional combustion exhaust gastreatment system (capable of conveying the dust from the electrostaticprecipitator in bath), the combustion exhaust gas treatment system canbe realized without modifying the conveyor for conveying the dust fromthe electrostatic precipitator and other structures, and the Se in thecombustion exhaust gas is made harmless, and modification of theexisting combustion exhaust gas treatment system is easy, and when newlyinstalling this system, the conventional design or equipment may be usedas it is.

The result of dust heating experiment, dust capturing test, and elutionexperiment by the same apparatus as in the embodiment is describedbelow.

In the experiment, coal containing 3 mg/kg of Se was supplied into acombustion furnace at a rate of 25 kg/h, and the combustion exhaust gasexhausted from the combustion furnace at a rate of 200 m³ N/h was cooledto 150° C. and fed into the electrostatic precipitator. In this case,more than 99% of the dust was captured by the electrostaticprecipitator, and the amount of dust collected by the conveyor (thetotal collected from the hoppers) was 3.4 kg/h. The capturing amount ofcoarse ash B5 and fine ash B6, mean particle size, and eluting Seconcentration are shown in Table 10.

TABLE 10 Collected ash Item Coarse ash B5 Fine ash B6 Capturing amountkg/h 2.05 1.30 Mean particle size of captured ash 13 5.4 μm Elutingselenium concentration in 0.26 0.36 captured ash mg/liter

That is, in the fine ash B6, the mean particle size was 5.4 μm, and thecapturing amount was small, 1.30 kg/h, but the eluting Se concentrationexceeded the standard, 0.36 mg/liter. In coarse ash B5, the meanparticle size was 13 μm, the capturing amount was large, 2.0 kg/h, butthe eluting Se concentration was below the standard, 0.26 mg/liter.Accordingly, it is known that the coarse ash B5 can be directlydiscarded. Hence, Se insoluble treatment is not needed in the coarse ashB5 which is very large in output, and it is evident that the requiredamount of treating agent A and required capacity of heating means 411can be saved substantially.

The fourth invention may be also realized in many other forms aside fromthe foregoing embodiments. For example, if hexavalent Se does not existand only other Se than hexavalent is present in the combustion exhaustgas in the desulfurization apparatus, the process or apparatus forreducing hexavalent Se into tetravalent Se is not needed.

The constitution of the desulfurization apparatus is not limited to thetank oxidation type shown in the embodiments, and, for example, anoxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

In the embodiments, as mentioned above, the charging position of thetreating agent A may be a position in the wastewater treating apparatus450. That is, since the slurry liquid in the absorption column orcooling and dust collecting column of the desulfurization apparatus iscirculating in the wastewater treating apparatus 450, the entire Se canbe made insoluble by charging the treating agent only in the wastewatertreating apparatus 450.

As the treating agent for making tetravalent Se insoluble, for example,aside from FeCl₃, Fe₂ (SO₄)₃, chelating agent (e.g. Epolus MX-7 ofMiyoshi Resin), or high molecular heavy metal capturing agent (e.g.Epofloc L-1 of Miyoshi Resin) may be used.

Effects of the Fourth Invention

According to the combustion exhaust gas treatment system of the fourthinvention (1), most of Se in flue is removed by the dust collector in astate being contained in the dust, and is heated and gasified by theheating means. Accordingly, almost no Se is left over in the dust aftertreatment, which can be directly recycled or discarded, and the Seelution standard is easily satisfied.

According to the combustion exhaust gas treatment system of the fourthinvention (2), the Se removed together with the dust from the combustionexhaust gas is heated and gasified, and introduced into thedesulfurization apparatus, and eventually all of hexavalent Se can betreated as tetravalent Se, and only by insoluble treatment by treatingagent, the Se elution standard is easily satisfied, and moreover theabsorption column of the desulfurization apparatus also functions as theSe reducing reaction facility, and hence the constitution of the entiresystem is simple as compared with the constitution for providing thereaction column for reducing Se separately. Still more, owing to theconstitution designed for separating Se from the dust by heating meansand feeding into the desulfurization apparatus, so that the entire dustmay not fed into the desulfurization apparatus, recycling of dust iseasy, and lowering of desulfurization performance in the desulfurizationapparatus can be avoided.

According to the combustion exhaust gas treatment system of the fourthinvention (3), the oxidation-reduction reaction control means controlsthe oxidation-reduction reaction of the slurry in the desulfurizationapparatus, so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost entirely reduced by thesulfurous acid in the slurry to be tetravalent. Hence, the hexavalent Secan be almost completely transformed into tetravalent form in thedesulfurization apparatus, and Se insoluble treatment in the combustionexhaust gas is done more easily and perfectly.

According to the combustion exhaust gas treatment system of the fourthinvention (4), the Se elution standard can be satisfied easily, and thedesulfurization apparatus functions also as hexavalent Se reducingreaction facility, and therefore the constitution of the entire systemis simplified as compared with the constitution for comprising reactioncolumn for reducing Se separately. In this case, the gasified Se andcombustion exhaust gas are fed and treated in the cooling and dustcollecting column in the desulfurization apparatus, and not only dustbut also impurities such as Se may not be mixed passively into theslurry in the absorption column of the desulfurization apparatus, and.therefore the desulfurization rate in the desulfurization apparatus ismaintained high, and gypsum of high quality can be obtained.

According to the combustion exhaust gas treatment system of the fourthinvention (5), only the dust separated and collected from the specificrecovery unit at the outlet side of the combustion exhaust gas in thedust collecting means is heated, and only the gasified Se is fed intothe desulfurization apparatus to be made insoluble, and therefore therequired amount of treating agent and required capacity of heating meansmay be reduced, so that the Se may be made harmless more easily andeconomically.

According to the combustion exhaust gas treatment system of the fourthinvention (6), only the dust of small particle size sorted by thesorting means is heated, and only the gasified Se is fed into thedesulfurization apparatus to be made insoluble, and therefore therequired amount of treating agent and required capacity of heating meansmay be reduced, so that the Se may be made harmless more easily andeconomically.

According to the combustion exhaust gas treatment system of the fourthinvention (6), the heating temperature of the dust by the heating meansis 100 to 1200° C., and hence the gasified Se will not be condensedagain in the dust, so that the Se can be removed efficiently from thedust, and therefore the Se elution standard of dust can be satisfiedeasily.

E: Fifth Invention

Embodiments of the fifth invention are described below by reference tothe drawings.

Embodiment 1

Embodiment 1 of the fifth invention (1) and (3) is described in thefirst place.

FIG. 20 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system in embodiment 1 of the fifthinvention. Constituent elements same as in the prior art are identifiedwith same reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of this embodiment, as shown inFIG. 20, circulation liquid B (liquid discharged from a pump 534described below) extracted from a wet combustion exhaust gasdesulfurization apparatus 520 (hereinafter desulfurization apparatus520) is mixed with treating agent A by mixing means 513, and thecirculation liquid B mixed with the treating agent A is sprayed by apump 514 from a spray pipe 516 a or 516 b. provided in a combustionexhaust gas lead-in passage 515 upstream of an electrostaticprecipitator 505, and the. Se contained in the dust is made insoluble bythe treating agent A, discharged as dust E, while the wastewaterdischarged from the desulfurization apparatus 520 is evaporated toeliminate the wastewater. Herein, the pump 514, spray pipe 516 a or 516b, and the piping system for connecting them compose the spray means ofthe fifth invention.

In this embodiment, since the electrostatic precipitator 505 isprovided, as described below, most Se in combustion exhaust gas isremoved as being contained in the dust, and very little Se mixes intothe desulfurization apparatus 520, but in the desulfurization apparatus,as shown in reaction formulas 9 and 10, there are tetravalent Se (mainform: selenious acid SeO₃ ²⁻) and hexavalent Se (main form: selenic acidSeO₄ ²⁻). In this constitution, the hexavalent Se is almost completelyreduced by the so-called ORP control (oxidation-reduction potentialcontrol) in the desulfurization apparatus 520, and is transformed intotetravalent Se.

SeO_(2(g))+H₂O2H⁺+SeO₃ ²⁻  (9)

SeO₃ ²⁻+½O₂→SeO₄ ²⁻  (10)

The mixing means 513 is composed of, for example, a mixing tank, and anagitating mechanism for agitating the liquid in the mixing tank. Thecirculation liquid B extracted from the desulfurization apparatus 520and the treating agent A are charged and mixed, and extracted by thepump 514, and sent into the spray pipe 516 a or 516 b. Herein, as thetreating agent, a chemical at least reacting with tetravalent Se (mainform: selenious acid SeO₃ ²⁻) to make it insoluble is needed, and forexample, FeCl₃ or Fe₂ (SO₄)₃ may be used.

The spray pipe 516 a or 516 b is composed of a pipe main body installedin the combustion exhaust gas lead-in passage (duct) 515 and connectedto the pump 514, and a spray nozzle formed in the pipe main body. Thelocation of the spray pipe 516 a or 516 b (the position for spraying theliquid) in FIG. 20 is in the upstream of a heat recovery unit 504 and inthe upstream of the electrostatic precipitator 505, but it may be alsoinstalled in the upstream side of an air heater, not shown, installed inthe upstream side of the combustion exhaust gas lead-in passage 515, asfar as the sprayed liquid may splash in the combustion exhaust gas andbe gasified by the heat of the combustion exhaust gas, and the treatingagent A in the liquid may efficiently contact with the dust in thecombustion exhaust gas.

The desulfurization apparatus 520 is of tank oxidation type, andcomprises an absorption column 521 for feeding an. absorbent slurry(composed of limestone in this example) into a bottom tank 522, acirculation pump 523 for sending the absorbent slurry in the tank 522into an upper part 521 a (combustion exhaust gas feed-in unit) of theabsorption tank 521 to contact with the combustion exhaust gas, a rotaryarm type air sparger 524 supported in the tank 522 for rotationhorizontally by means of a motor not shown, and agitating the slurry inthe tank 522 and blowing in the supplied aid efficiently into the tank522 as fine bubbles, and an air feed tube 525 for feeding air into thisair sparger 524, and it is designed to obtain gypsum by totallyoxidizing by efficient contact between air and the absorbent slurryabsorbing sulfurous acid in the tank 522.

A slurry pump 531 for sucking out the slurry in the tank 522 isconnected to the tank 522, and the slurry sucked out by this slurry pump531 is supplied into a solid-liquid separator 532 to be separated intosolid and liquid, and gypsum C in the slurry is taken out as solid cake(usually water content of about 10%). On the other hand, the separatedliquid (mainly water) from the solid-liquid separator 532 are once sentinto a separated liquid tank 533, and, as required, makeup water D isadded, and part of such liquid is sent into an absorbent slurry tank 535by a pump 534, and mixed with limestone F (CaCO₃) supplied from alimestone silo not shown to be formed into an absorbent slurry, which issupplied again into the tank 522 by a slurry pump 536.

The desulfurization apparatus 520 is further provided with, as apreferred embodiment of the fifth invention, oxidation-reductionreaction control means 540 for controlling the oxidation-reductionreaction in the absorption column 521. This oxidation-reduction reactioncontrol means 540 is composed of a sensor 541 disposed in the dischargeside piping of the circulation pump 523 for detecting theoxidation-reduction potential of the slurry in the tank 522, a flow ratecontrol valve 542 disposed in the midst of the air feed tube 525 foradjusting the air feed rate into the air sparger 524, and a controller543 for controlling the action of the flow rate control valve 542 on thebasis of the detection output of the sensor 541. Herein, the sensor 541is composed by immersing an electrode, for example, made of platinuminto slurry. The controller 543 is designed to control the openingdegree of the flow rate control valve 542 continuously, so that the airfeed rate into the air sparger 524 may be a minimum required limit foroxidizing and digesting the sulfurous acid dissolved in the slurry fromthe combustion exhaust gas. For example, more specifically, on the basisof the correlation of the concentration of sulfurous acid andoxidation-reduction potential, the oxidation-reduction potential whenthe concentration of sulfurous acid is nearly zero is predetermined asthe reference potential, and, by proportional control, when theoxidation-reduction potential detected by the sensor 541 becomes lowerthan this reference potential, the air feed rate is increased accordingto the deviation, and when the oxidation-reduction potential detected bythe sensor 541 becomes higher than this reference potential, the airfeed rate is decreased according to the deviation.

Incidentally, since the oxidation-reduction reaction control means 540is designed to feed a minimum required limit for oxidizing the totalvolume of sulfurous acid, it eventually has a function of inducing anearly total reduction reaction of the other acids contained in theslurry by the sulfurous acid.

That is, the slight Se not removed by the electrostatic precipitator 505is fed into the absorption column 521 together with combustion exhaustgas, and tetravalent Se (main form: selenious acid SeO₃ ²⁻) andhexavalent Se (main form: selenic acid SeO₄ ²⁻) exist, and thehexavalent Se is controlled by the controller 543, and reacts with thesulfurous acid absorbed from the combustion exhaust gas to undergoreduction reaction to be transformed into tetravalent Se (main form:selenious acid SeO₃ ²⁻), which takes place in the absorption column 521.This reaction is expressed in the reaction formula (11).

SO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SeO₄ ²⁻  (11)

In thus constituted combustion exhaust gas treatment system, in theupstream of the electrostatic precipitator 505, the combustion exhaustgas is sufficiently cooled, and most Se in the combustion exhaust gas iscondensed and deposits on the fly ash and other dust (of smallerparticle size in particular), and hence most of Se in the combustionexhaust gas is captured by the electrostatic precipitator 505 togetherwith the dust. Moreover, at the upstream side of the electrostaticprecipitator 505, the liquid containing treating agent A is sprayed fromthe spray pipe 561 a or 516 b, and reacts with Se depositing on the dustin the combustion exhaust gas. Accordingly, tetravalent Se (main form:selenious acid SeO₃ ²⁻) almost completely reacts as shown in reactionformulas (1), (2), or (3), (4), and becomes iron selenite (Fe₂ (SeO₃)₃)to be insoluble, and is mixed in the removed dust.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ³⁻→Fe₂(SeO₃)₃↓(4)

In this case, the Se in the captured dust E is made insoluble by thetreating agent A, and can be directly recycled as the cement material ordiscarded, while the Se elution standard is satisfied.

On the other hand, the combustion exhaust gas fed into the absorptioncolumn 521 contacts with the absorbent slurry. sprayed from the spraypipe 526 by the circulation pump 523, and sulfurous acid and Se areabsorbed and removed, and the treated combustion exhaust gas isdischarged from a combustion exhaust gas lead-out unit 521 b.

The sulfurous acid absorbed in the absorbent slurry sprayed from thespray pipe 526 and flowing down through a filler 527 is agitated by theair sparger 524 in the tank 522, and contacts with multiple bubblessucked in to be oxidized, and further undergoes neutralization reactionto become gypsum. In the absorption column 521, by the reaction in thereaction formula (11), nearly whole volume of hexavalent Se (main form:selenic acid SeO₄ ²⁻) is transformed into tetravalent Se (main form:selenious acid SeO₃ ²⁻). Principal reactions taking place in thisprocess (other than reaction formula (11)) are expressed in reactionformulas (15) to (17).

Absorption Column Combustion Exhaust Gas Lead-in Part

SO₂+H₂O→H⁺+HSO₃ ⁻  (15)

Tank

H⁺+HSO₃ ⁻+½O₂→2H+SO₄ ²⁻  (16)

2H⁺+SO₄ ²⁻+CaCO₃+H₂O→CaSO₄.2H₂O+CO₂  (17)

Thus, in the tank 522, gypsum (CaSO₄.2H₂O), a slight amount of limestone(CaCO₃), and tetravalent Se (main form: selenious acid SeO₃ ²⁻) aresuspended or dissolved, and they are sucked out by the slurry pump 531,and supplied into the solid-liquid separator 532 to be separated intosolid and liquid, and is taken out as gypsum C in a cake form of lowwater content (usually water content about 10%). At this time, if notdissolved partly, tetravalent Se (main form: selenious acid SeO₃ ²⁻) maybe slightly mixed into the separated gypsum C, but the majority is sentinto the separated liquid tank 533 together with the separated liquid.

The liquid in the separated liquid tank 533 is. blended with, ifnecessary, makeup water D as mentioned above, and partly sent into theabsorbent slurry tank 535 by the pump 534 to be mixed with limestone F,and is supplied again as absorbent slurry into the tank 522 tocirculate, and in this case, further, part of the liquid in theseparated liquid tank 533 is sent into the mixing means 513, and blendedwith the treating agent A, and sprayed into the combustion exhaust gas.

Accordingly, as mentioned above, the Se mainly composed of tetravalentform fed into the desulfurization apparatus 520, and other impurities(Cl, etc.) mixing into the circulation liquid in the desulfurizationapparatus from the combustion exhaust gas are prevented from beingaccumulated excessively in the circulation liquid system in thedesulfurization apparatus 520, and hence the high desulfurizationperformance and gypsum quality are not impaired, which eliminates therequirement of installation of wastewater treating apparatus (composedof electric dialysis machine, etc.) for the desulfurization apparatus.That is, part of the circulation liquid in the desulfurization apparatus520 is extracted sequentially as stated above, and blended with thetreating agent, then sprayed into the combustion exhaust gas, andtherefore the tetravalent Se in the circulation liquid is sequentiallymade insoluble, and is removed by the electrostatic precipitator 505 asbeing contained in the dust., together with the tetravalent Se in thecombustion exhaust gas newly introduced from the lead-in passage 515. Atthe same time, other impurities mixing in the circulation liquid aresent into the combustion exhaust gas lead-in passage 515 in the route ofmixing means 513 and pump 514, and returned to the. combustion exhaustgas, and hence they are partly removed by the electrostatic precipitator505 together with the dust, or pass through the absorption column 521 tobe discharged from the combustion exhaust gas lead-out unit 521 btogether with the treated combustion exhaust gas, so that the may not beaccumulated in the circulation liquid in the desulfurization apparatus520.

As described herein, according to the combustion exhaust gas treatmentsystem of embodiment 1, aside from the conventional combustion exhaustgas purification (removal of dust, removal of sulfurous acid), most ofSe in the combustion exhaust gas can be contained in the dust in aninsoluble form, so that it may be directly recycled or discarded. Whatis more, the hard-to-treat (make insoluble) hexavalent Se is transformedinto an easy-to-discard tetravalent Se by the treating agent by theoxidation-reduction reaction control means 540 in the absorption column521 of the desulfurization apparatus 520, and therefore, as comparedwith the system comprising an independent reaction column fortransforming hexavalent Se into tetravalent Se, for example, the Se inthe combustion exhaust gas can be removed and made harmless by a simpleand inexpensive system.

According to this combustion exhaust gas treatment system, moreover,almost all of hexavalent Se is eventually transformed into tetravalentSe in the absorption column 21 by the function of theoxidation-reduction reaction control means 540, and is finally madeinsoluble and discarded, and therefore the concentration of theremaining hexavalent Se (not made insoluble) is extremely slight, andthe elution standard may be satisfied with an ample margin.

In this case, furthermore, since the circulation liquid discharged fromthe desulfurization apparatus 520 is sprayed into the combustion exhaustgas lead-in passage, without particularly requiring the wastewatertreating apparatus as mentioned above, impurities including Se areprevented from being accumulated excessively in the circulation liquidin the desulfurization apparatus 520, and the water in thedesulfurization can be treated in a simple constitution, and the systemcan be further reduced in cost and size, while the. desulfurizationperformance and gypsum quality may be maintained high.

Embodiment 2

Embodiment 2 of the fifth invention (1) is described below. FIG. 21 is aschematic structural diagram showing a constitution of a combustionexhaust gas treatment system of embodiment 2. In FIG. 21, sameconstituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

In the combustion exhaust gas treatment system of this embodiment, asshown in FIG. 21, a desulfurization apparatus 560 comprising a coolingand dust collecting column 561 for cooling the flue and collecting dustis disposed at the upstream side of an absorption column 521, andcombustion exhaust gas containing Se and others is fed into the coolingand dust collecting column 561 of the desulfurization apparatus 560, andthe liquid extracted from the cooling and dust collecting column 561 isfed into mixing means 513 as discharge circulation liquid B.

In the cooling and dust collecting column 561, the liquid in a separatedliquid tank 533 is supplied by a pump 534, and this liquid is sprayedfrom an upper header pipe 563 by a circulation pump 562. Between thecooling and dust collecting column 561 and the absorption column 521, amist eliminator, not shown, is provided.

In this constitution, impurities including Se getting into thedesulfurization apparatus 560, that is, fine dust particles not capturedby the electrostatic precipitator hardly mix into the absorption column521, but are mainly absorbed in the cooling and dust collecting column561, and are extracted as being contained in the circulation liquid B,and consequently mixed with the treating agent A and sprayed into thecombustion exhaust gas lead-in passage 515 same as in embodiment 1.

Therefore, in addition to the action and effect of embodiment 1, it isfurther effective to maintain a high desulfurization rate and a highquality (purity) of gypsum C.

Embodiment 3

Embodiment 3 of the fifth invention (2) is described below. FIG. 22 is aschematic structural diagram showing a constitution of a combustionexhaust gas treatment system of embodiment 3. In FIG. 22, sameconstituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

In the combustion exhaust gas treatment system of this embodiment, asshown in FIG. 22, without installing electrostatic precipitator, it ischaracterized by feeding the combustion exhaust gas containing fly ashand dust directly into the absorption column 521 of the desulfurizationapparatus 520.

In this case, most of Se in the combustion exhaust gas is contained inthe dust, and enters the absorption column 521 entirely, and at leasttetravalent Se thereof reacts with the sprayed treating agent A in thecombustion exhaust gas lead-in passage 515 to be insoluble, and isseparated in the solid-phase side by the solid-liquid separator 532, andis mixed into the gypsum C in insoluble state. If hexavalent Se ispresent, it is transformed into tetravalent Se in the absorption column521 same as in embodiment 1, and is sequentially extracted as beingcontained in the discharge circulation liquid B, and is mixed with thetreating agent A and sprayed into the combustion exhaust gas lead-inpassage 515, and fed again into the desulfurization apparatus 520, andultimately, therefore, almost all of Se is transformed into an insolubleform as tetravalent Se and is mixed in the gypsum C.

In this case, the absorption column 521 also functions as theelectrostatic precipitator 505 in embodiment 1, and hence the facilitycost may be lower than in the system of embodiment 1.

In this embodiment, as compared with embodiment 1, the cost may befurther lowered, but due to the effects of the dust (impurities)massively mixing into the absorption column, it may be difficult torealize high desulfurization rate or high quality of gypsum C, and ifthis is a problem, it is preferred to constitute as in embodiment 1 or 2or embodiment 4 described below.

Incidentally, the dust E discharged from the heat recovery unit 504 isslight, and the Se contained in the dust E is made insoluble by thetreating agent A, and hence it may be discarded directly.

Embodiment 4

Embodiment 4 of the fifth invention (3) is described below. FIG. 23 is aschematic structural diagram showing a constitution of a combustionexhaust gas treatment system of embodiment 4. In FIG. 23, sameconstituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

In the combustion exhaust gas treatment system of this embodiment, asshown in FIG. 23, a desulfurization apparatus 560 comprising a coolingand dust collecting column 561 for cooling the flue and collecting dustis disposed at the upstream side of an absorption column 521, and thecombustion exhaust gas not deprived of dust is directly fed into thecooling and dust collecting column 561 of the desulfurization apparatus560, and the dust slurry extracted from the cooling and dust collectingcolumn 561 is separated into solid and liquid in the separating means517, and the separated liquid discharged from the separating means 517is led into the mixing means 513. The separating means 517 is, forexample, composed of a centrifugal settling machine, and it dischargesthe dust slurry led into the dust in the cooling and dust collectingcolumn 561 as cake of low water content (dust cake G).

In this case, most of Se in combustion exhaust gas is fed into thecooling and dust collecting column 561 as being contained in the dust,and at least tetravalent Se thereof reacts with the sprayed treatingagent A in the combustion exhaust gas lead-in passage 515 to beinsoluble, and it is separated in the solid phase side in the separatingmeans 517, and is mixed in the dust cake G in insoluble state. Ifhexavalent Se is present, it is transformed into tetravalent Se in thecooling and dust collecting column 561 same as in embodiment 2, and issequentially extracted as being contained in the discharge slurry, andis blended with the treating agent A and sprayed into the combustionexhaust gas lead-in passage 515, and is led into the desulfurizationapparatus 520 again, and finally almost all Se is made insoluble astetravalent Se, and is mixed in the dust cake G. Therefore, in thiscase, too, if the dust cake G is directly discarded, the Se elutionstandard is satisfied.

Moreover, the cooling and dust collecting column 561 also function asthe electrostatic precipitator 505 in embodiment 2, and it is effectiveto reduce the cost of equipment as compared with the system ofembodiment 2. Still more, in this embodiment, different from embodiment3, much dust does not mix into the absorption column 521, and hence theequipment cost may be further decreased, while high desulfurization rateand high quality of gypsum C may be realized.

The fifth invention is not limited to the illustrated embodiments alone,but various modifications are possible. For example, .if hexavalent Sedoes not exist and only other Se than hexavalent is present in thecombustion exhaust gas in the desulfurization apparatus, the process orapparatus for reducing hexavalent Se into tetravalent Se is not needed.

The constitution of the desulfurization apparatus is not limited to thetank oxidation type shown in the embodiments, and, for example, anoxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

As the treating agent for making tetravalent Se insoluble, for example,aside from FeCl₃, Fe₂ (SO₄)₃, chelating agent (e.g. Epolus MX-7 ofMiyoshi Resin), or high molecular heavy metal capturing agent (e.g.Epofloc L-1 of Miyoshi Resin) may be used.

Effects of the Fifth Invention

According to the combustion exhaust gas treatment system of the fifthinvention (1), at least the tetravalent Se in the combustion exhaust gasreacts with the treating agent sprayed into the combustion exhaust gaslead-in passage by the spraying means and becomes insoluble before it isremoved by the dust collector. Accordingly, at least the content ofother Se than tetravalent Se is less, if the dust after treatment isdirectly recycled or discarded, the Se elution standard is satisfied.

Incidentally, if Se or other impurities may be mixed slightly into thecirculation liquid in the desulfurization apparatus without beingremoved by the dust collector, most of Se becomes tetravalent, and iscontained in the circulation liquid together with other impurities bythe function of the mixing means and spraying means, and extracted,blended with treating agent, and sprayed into the combustion exhaust gaslead-in passage. Accordingly, Se and impurities mixing into thecirculation liquid of the desulfurization apparatus mix into the dust tobe removed by the dust collector, and are sequentially discharged,thereby preventing excessive accumulation of these impurities into thecirculation liquid in the desulfurization apparatus. Therefore, notrequiring to install wastewater treating apparatus for thedesulfurization apparatus, the system may be reduced in cost and size,while the desulfurization performance in the desulfurization apparatusand byproduct purity may be maintained high.

According to the combustion exhaust gas treatment system of the fifthinvention (2), although most Se in the combustion exhaust gas isabsorbed in the desulfurization apparatus, at least tetravalent Sethereof reacts with the treating agent sprayed into the flue lead-inpassage to be insoluble, and is discharged as being mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus. The hexavalent Se is transformed intotetravalent Se by reduction reaction in the desulfurization apparatus,and is contained in the circulation liquid in the desulfurizationapparatus, and is sequentially extracted together with other impurities,and is sprayed into the combustion exhaust gas lead-in passage togetherwith the treating agent, so that it is finally discharged as being mixedin the solid matter (gypsum, etc.) separated and formed in thedesulfurization apparatus, and thereby Se and other impurities are notaccumulated in the circulation liquid.

Therefore, by this system, too, the Se elution standard may be satisfiedeasily, and moreover the desulfurization performance of thedesulfurization apparatus and byproduct purity can be maintained highwithout using wastewater treating apparatus.

According to the combustion exhaust gas treatment system of the fifthinvention (3), although most Se in the combustion exhaust gas isabsorbed in the cooling and dust collecting column of thedesulfurization apparatus, at least tetravalent Se thereof reacts withthe treating agent sprayed by the spraying means into the flue lead-inpassage to be insoluble, and is discharged as being mixed in the solidmatter (dust cake) separated and formed by the separating means forseparating the dust slurry in the cooling and dust collecting columninto solid and liquid. The hexavalent Se is mostly transformed intotetravalent Se by reduction reaction with the sulfurous acid absorbed inthe liquid in the cooling and dust collecting column, and is containedin the filtrate in the separating means, and is sequentially extractedtogether with other impurities, and is sprayed into the combustionexhaust gas lead-in passage together with the treating agent, so that itis finally discharged as being mixed in the solid matter separated andformed by the separating means, and thereby Se and other impurities arenot accumulated in the circulation liquid.

Therefore, by this system, too, the Se elution standard may be satisfiedeasily, and moreover the desulfurization performance of thedesulfurization apparatus and byproduct purity can be maintained highwithout using wastewater treating apparatus. Moreover, in this system,since almost no impurity such as dust mixes into the slurry in theabsorption column in the desulfurization apparatus, the desulfurizationrate in the desulfurization apparatus, gypsum purity and otherperformances may be maintained high.

According to the combustion exhaust gas treatment system of the fifthinvention (4), if hexavalent Se is present, this hexavalent Se may bealmost completely transformed into tetravalent Se in the desulfurizationapparatus, and hence the Se in the combustion exhaust gas may be treatedmore easily and perfectly.

What is claimed is:
 1. A combustion exhaust gas treatment apparatus fortreating combustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dry dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having adesulfurization column in which an absorbent slurry for absorbing andremoving sulfurous acid circulates, means for introducing the dustremoved by the dry dust collector into the absorbent slurry, andtreating agent feeding means for feeding a treating agent for makingtetravalent Se insoluble into the absorbent slurry.
 2. A combustionexhaust gas treatment apparatus of claim 1, further comprisingoxidation-reduction reaction control means for controlling theoxidation-reduction reaction in the desulfurization apparatus, so thatthe hexavalent Se in the slurry in the desulfurization apparatus may bereduced to tetravalent, by the sulfurous acid in the slurry, wherein thetreating agent is selected from the group consisting of FeCl₃,Fe₂(SO₄)₃, chelating agent and a resin agent for capturing heavy metal.3. A combustion exhaust gas treatment apparatus of claim 2, furthercomprising a wastewater treatment apparatus for treating wastewater fromthe desulfurization apparatus, and treating agent means for feeding atreating agent into the impurity slurry separated by this wastewaterapparatus, for making tetravalent Se insoluble.
 4. A combustion exhaustgas treatment apparatus of claim 1, further comprising a wastewatertreatment apparatus for treating wastewater from the desulfurizationapparatus, and treating agent means for feeding a treating agent intothe impurity slurry separated by this wastewater apparatus, for makingtetravalent Se insoluble.
 5. A combustion exhaust gas treatmentapparatus of claim 1, further comprising a wastewater treatmentapparatus for treating wastewater from the desulfurization apparatus,and treating agent means for feeding a treating agent into the impurityslurry separated by this wastewater apparatus, for making tetravalent Seinsoluble.
 6. A combustion exhaust gas treatment apparatus for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having adesulfurization column in which an absorbent slurry for absorbing andremoving sulfurous acid in the combustion exhaust gas circulates, andtreating agent feeding means for feeding a treating agent into theabsorbent slurry for making tetravalent Se insoluble, wherein thecombustion exhaust gas is introduced directly into the desulfurizationcolumn, wherein the treating agent is selected from the group consistingof FeCl₃, Fe₂(SO₄)₃, chelating agent and a resin agent for capturingheavy metal.
 7. A combustion exhaust gas treatment apparatus of claim 6,further comprising a wastewater treatment apparatus for treatingwastewater from the desulfurization apparatus, and treating agent meansfor feeding a treating agent into the impurity slurry separated by thiswastewater apparatus, for making tetravalent Se insoluble.
 8. Acombustion exhaust gas treatment apparatus for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dry dust collector for removing dust in the combustionexhaust gas, a desulfurization apparatus having a cooling and dustcollecting column disposed upstream of an absorption column and adesulfurization column in which an absorbent slurry for absorbing andremoving sulfurous acid circulates, means for feeding the dust into acirculating slurry in the cooling and dust collecting column, and meansfor feeding a treating agent into the circulating slurry in the coolingand dust collecting column, for making tetravalent Se insoluble.
 9. Acombustion exhaust gas treatment apparatus of claim 8, furthercomprising a wastewater treatment apparatus for treating wastewater fromthe desulfurization apparatus, and treating agent means for feeding atreating agent into the impurity slurry separated by this wastewaterapparatus, for making tetravalent Se insoluble.
 10. A combustion exhaustgas treatment apparatus for treating combustion exhaust gas containing asulfurous acid, dust and Se components, comprising a desulfurizationapparatus having a cooling and dust collecting column disposed upstreamof an absorption column and a desulfurization column in which anabsorbent slurry for absorbing and removing sulfurous acid circulates,and means for feeding a treating agent into the circulating slurry inthe cooling and dust collecting column, for making tetravalent Seinsoluble, wherein the combustion exhaust gas is introduced directlyinto the cooling and dust collecting column, wherein the treating agentis selected from the group consisting of FeCl₃, Fe₂(SO₄)₃, chelatingagent and a resin agent for capturing heavy metal.
 11. A combustionexhaust gas treatment apparatus of claim 10, further comprising awastewater treatment apparatus for treating wastewater from thedesulfurization apparatus, and treating agent means for feeding atreating agent into the impurity slurry separated by this wastewaterapparatus, for making tetravalent Se insoluble.